So I am helping out with rebuilding an XT600 engine after the slightly dramatic self-destruction you saw recently. As a matter of fact, as I inherited the Dre-XT-Stück myself a bit of re-familiarization with the XT600 engine won't exactly do me harm, mainly because I want the Dre-XT-Stück to ultimately grow into a 600.
If there's one golden rule to adhere, when someone gives you an engine in parts. Never believe anything until you've proven it yourself. In this particular case it meant cleaning everything even down to the oilpump strainer mesh and I found: Not much. Bit of dust from this engine sitting in a shop for at least a year or two.
Guess what? Some people have all the look and find an XT600 gearbox with no discernible wear on it for 100 Euros.
As the engine came with a load of seals and o-rings in original Yamaha bags AND there was a lot of oil around the gearbox selector seal, this one was quickly swapped.
Cases testfitted with the gearbox in. All seems to be there AND working. Unfortunately I couldn't permanently button it up as the gasket between the oilpump strainer and the engine case was missing and I didn't want to use RTV there.
So once the missing gasket has arrived the cases will be RTV'ed and bolted together and then it's really just a matter of throwing some more parts on it and see what sticks and what doesn't.