, these motors are bullet proof, and mostly splash lube low pressure oiling systems anyway.
Unfortunately these twins are plain bearing engines.
Before 1980 they had all the benefits of the larger 4 cylinders.
On 4 cylinder Roller bearing engines, they are much more forgiving.
All GS/X 4-5xx are plain bearing engines after 80.
What would fit ??
1980 + gs 400. 450, 500 BASE with any head you want to use as long as its newer than 80. Even your own if you find its still within specification.
The GS500 is a direct bolt in but you need the carbs to go with it and you need to manufacture an exhaust system.
Another gs 450 would be best.
If you want to fix it, start with the simple things.
1 take a video.
2 post it wherever
3 link it here and we can help narrow it down.
4 dont run it longer than necessary.
After that depending on the noise its making,
It could be
Main bearings: they are the first to get oil and at the highest pressure directly from the pump unrestricted. Oil from there then goes to the connecting rod big ends.
The cylinder and small end get splashed with oil coming out of the connecting rod big end.
At the same time oil is going to the balance shaft but it is restricted.
And then to the clutch shaft bearings which are also restricted.
And the most restricted and last in line to get oil is the top end.
SO with lack of lubrication my thoughts on what failed first would be:
Top end.
Main crank bearings (Makes a deep knock sound)
Connecting rod bearings ( makes a faster sharper knocking sound)
Balance shaft baring ( similar to main bearing failure)
All these are plain bearings by the way.
the clutch shaft probably survived.
I will shut up now and stick to what I know.
