how in depth are you looking to go on your build? If you have any questions regarding your performance brake, suspension, engine, or frame modifications, I have been in and out of many of these bikes in the past 6 years now, and am basically obsessed with them, so don't be a stranger, email or PM me!
not sure what your plans are other than the look you are going after, but the top priorities are getting the mechanicals in good order. Once rideable, don't push it until you get a good set of very grippy tires. Avon am26 road riders if you plan to put on lots of miles on straight flat roads. Otherwise Shinko 230 tour master is the best gripping tire for a low price, 100/90-19 ft & 110/90-18 rear are the absolute biggest you want to stuff onto the stock rims. it will handle worse with bigger tires than that, will be very floppy as you lean into turns and give you less straight line traction. Pirelli Sport Demons are the #1 best grippers, 90/90-19 front & 110/90-18 rear. phenomenal cornering ability. The Shinko tires are almost as good, 90% of the grip. Bridgestone battleax bt45 is a decent alternate, but for the money, go all the way with the Pirelli.
the carbs will probably hold you up from getting it to run good. Don't buy the cheap rebuild kits that you see all over, insist on only OEM or better parts, and the rebuild kits don't come with most of the o-rings that desperately need replaced on these old carbs. basically all you will typically want for a full professional rebuild is the complete o-ring kit, top and bottom gaskets, aluminum fuel tees if you do not already have them stock, and possibly needle and seat sets. member Robert Barr on here has cycleorings.com where he sells complete kits for these carbs. You will also find a download on the main page of the GS resources for a VM carb rebuild manual, and maybe on Roberts website as well. This is way more in depth than anything you will find in a commercial repair manual. make sure to soak them at least overnight want and then chase with aerosol carb cleaner and rinse with water, then blow out all passages again with aerosol carb cleaner and compressed air at high pressure.make sure to get the float heights set properly, and replace the needle and seats as a matched set with Mikuni only oem parts, or at least with a matched set from jetsrus.com or the awesome Z1 enterprises. if your carbs have the rubber fuel tees in between them instead of the aluminum tee and couplers with replaceable o rings, just cough up the money up front and avoid fuel leaks on to your hot engine by getting the aluminum tee & couplers for use with replaceable o rings that z1 sells.
Oh, and remove the intake boots from the cylinder head and replace the large o ring that seals them to the head. Replace the stubborn number 3 Phillips fasteners with stainless allen head bolts. Also, inspect the intake boots and very very carefully and make sure the rubber is not separating from the steel plate, and that there are no cracks in them. A vacuum leak in these will give you a whole lot of hassle and poor running. They are expensive, about $130 a set of 4, but are mandatory to have in good condition.
the other very important thing to do is get rid of that stock Suzuki regulator/rectifier. They are absolute garbage and will fry your stator if it is not already compromised. there are a million different 3 phase regulator rectifier that you can swap on, but the latest greatest thing is the newer series style technology that just disconnects the stator when the extra voltage is not needed, where as most regulator rectifiers use front style technology, where they rapidly cycling shorts all the unused unnecessary voltage directly to ground, which lets the stator run at full load nonstop and makes it run much hotter.
Late model Polaris RZR 800 reg-recs are the absolute hot ticket to put onto our bikes, unless you want to run heated grips or any high demand accessories. Then the Compu fire unit is the way to go. search eBay for the Polaris OEM number 4012941. you will find A couple year old unit off of a wrecked Polaris for around $50, and this is an absolute must. the manufacturer is Shindengen, model is the SH-775, but you will not find it on eBay under that number typically, so look for that Polaris OEM number. refer to numerous threads on here as to which is the ground and which is the 14 volt connection. The three terminals in a row are the stator inputs.
also on the early models like you have, to avoid running the stator incredibly hot, but as a byproduct adding a mile of extra wire and an extra switch and see you connectors to get dirty and cause problems, Suzuki sent one leg of the stator through the headlight switch and then to the regulator rectifier. This way if you are not using the headlight, it completely disconnected 1 phase.the original setup was garbage anyway, and only regulated one phase of the three, so you were always dumping 1 full phase of unregulated power straight into the battery and charging system. when upgrading to the SH 775, or any other high quality unit, abandon this headlight switch run.
also, make sure you go through the entire wiring harness and check every connector then, cleaning them, crimping loose connections and bullet connectors, and putting silicone dielectric grease on everything. Also a good idea to dismantle the ignition switch and spray with contact cleaner and lubricant, same with the kill switch. these will cause low voltage to your ignition coils and poor idle, not to mention additional strain everywhere. and make sure that you have great ground connections. Jim, posplayr, GS owner and aviation electronics engineer, insists on a single point ground as close to the battery negative as possible, running all grounds to this point.
also, get the Suzuki valve shim depressor tool and a good set of feeler gauges in the smallest increments that autozone sells. .001" to .003" or .02-.08mm are your valve to cam clearances. They will get tight and cause you to burn valves if you do not do regular adjustments. Sometimes they will get loose and cause a lot of noise, but typically will get tight. join the GS shim club that Ray GhostGS1 believe his name is runs on this forum. you can trade in your shims that no longer are the proper fit and get the correct ones. or watch eBay for a complete Suzuki valve shim kit for $150 or so. always try and have a spare top cover gasket on hand for valve adjustments. not a bad idea to buy a generic gasket kit to have on hand, just don't use the head and base gaskets and internal engine orings from them. OEM only.
if you are certain you will be keeping the stock swing arm (length), definitely upgrade to a 530 o ring chain kit from the one or elsewhere. it will make your bike run much smoother. The stocl longer link 630 chains are quite clunky and very heavy.
Tapered roller bearing steering conversions from all balls will make the steering much better at high speeds and much more durable for around $40 as well (loose balls are oem, antiquated). They also sell wheel bearing kits for our bikes for good prices, nothing upgrading there, just brand new bearings if needed. Grease your swingarm needle bearings as well, there may be a grease fitting there, but if not, definitely tear them apart and clean them, blow out, and repack.
that just about covers the first phase of mandatory work that any old GS will need. Dyna makes a nice Dyna-S ignition if you don't want to fool around with adjusting points and replacing them at regular intervals.
