Apparently, a challenge in LED marker lights (apart from an efficient circuit that doesn't overheat) is designing a light that meets federal safety requirements for visibility.
I think they have to do with light intensity and angles of visibility.
(He also has a GS 550 he should be taking care of...)
Brother Dogma is right, there are lots of federal requirements for turn signals as well as all the other lamps on any vehicle. (he is also right that I don't take good enough care of my GS)
To start with, federal requirements set the size of the illuminated area of the lamp. Since you are retrofitting the existing lamp, you are covered there as long as the whole lamp lights up like with the bulb.
You should also consider the color of the LEDs. Just using yellow LEDs is not necessarily right. Sometimes the right color is called amber. You need to get LEDs that are in the 590 nm dominant wavelength range give or take 5 nm. (if they are listed in peak wavelength, then steer for about 597 nm give or take 5 nm. Digi-key and Mouser and Newark and Farnell sell LEDs that are in the right color range. (you can probably find the right color at Radio Shack, but I haven't actually looked.
Now for the hard part. Depending on the type of lens your existing lamp has will determine what sort of LED you put behind it. Some lenses are designed to take the light from that skinny little filament of the bulb and spread it all over the road. LEDs don't make light in that same skinny strip, so it is diffcult to get things to work right. This lens type will often have ring shaped optics with lots of sharp edges trying to collect the light and spread it across the road. With this type of lens, you will have a tough time getting things right. Sometimes those LED bulb things will work OK in this situation, but I have yet to see one that really makes a nice looking lamp. (OK, so that is opinion) The alignment is difficult because they make a very large source compared to the filament of the bulb, so you need to bring them closer to the lens. There is a limit to how close you can get before you aren't filling the lens. There are times you just can't get things to work out with this type of lens. Essentially you will end up trying to put a single wide viewing angle LED in the center of those rings at about the same distance that the bulb filament was, but just a little closer. This adjustment is finicky and difficult, but can be done. I made a lamp once that filled the lens well, and made a decent pattern, but I didn't measure it to see if it was bright enough so I don't know if I made a legal lamp or not. Normally, this will take a high power LED that will need heat sinking and driver electronics.
Occasionally, the OEM lamp designer will chrome the inside of the lamp housing so it acts like a collimating reflector. (essentially makes all the light rays as close to parallel as possible when they hit the lens) This arrangement makes the lens design easier because they can use what is called a pillow optic. (a bunch of little pillows across the lens) If your lens is covered with pillow optics, things are looking up. You want to find LEDs with as narrow a viewing angle as possible to simulate those parallel rays as best you can. You will need to set the LEDS up in a package a good distance from the LENS but spread out enough to fill the lens with those near parallel rays. You need to put a little space between the lens and the LEDs and fill the housing with LEDs at the same time. So just getting the brightest LEDs and filling the housing could make too much light. The key here is to fill the housing with LEDS so the parallel rays fill the lens.
To test things out, it is best to keep one bulb lamp as a standard. Mount it on your bench in the same orientation as on the bike. Mount your attempt at an LED lamp next to it with maybe 6 inches between them. Light them both up and stand back about 30 to 50 feet. They should look about the same brightness at this distance. The next step is important, but is often overlooked in LED lamp retrofits. Walk to the left and the right at that same distance and continue to compare the brightness of the two lamps. The LED lamp should stay as bright as the bulb lamp as you walk to the sides. (up to a 30 degree angle, but if you can get out to 45 it is better) The wider the better, because this is visibilty for you intentions to turn. (we all know the importance of visibility on bikes) It may take a half dozen tries to get the LED lamp as bright as or brighter than the bulb lamp at all the viewing angles. Once you get it right, you can then change over your reference lamp to LEDs. If you have limited space, you can mount the lamps on a pivot and have an assistant rotate the lamps instead of you walking around them.
If you have a flat lens, putting several of the wide viewing angle LEDs (?60?) in there and a few of the narrower ones (?20?) could make for a decent lamp. This is easiest to figure out because if you need more light on the sides, you add wide viewing angle LEDs. If you need more light in the back, you add some of the narrower viewing angle LEDs. A flat lens is rare, so don't get your hopes up. 8)
To sum up, you need to get the right color, fill the lens with light, and make sure you get equivalent output at the back as well as the off angles. This isn't a guarantee that you will make a legal lamp, but police officers don't carry photometers to test light output. They just pull you over for what looks like a dim lamp if they want to give you a hard time looking for a larger infraction. The steps above will get light where you need it so others can see you, and making it the same brightness or a little brighter than the bulb will make sure you are in the right ballpark.
For a personal bike modification, getting them to legal numbers is not a requirement, you just need to get them equivalent in appearance. The problem comes in if you are making them for profit. (liability requires that you do your homework and get things tested to prove you are making a legal lamp, otherwise you are risking a lawsuit)
Sorry for the verbose post, this is a complex topic that requires a lot of 'splainin
