I recently picked up a 1979 YZ250 Too! I cant wait to get out on it.
Im not completely sure about the trans but my guess would be that you would have to swap the parts from a known good unit. The Bike bandit site says that the parts are discontinued. You might find some NOS parts on e b a y or something but thats pretty rare.
As far as common problems, i have not heard much bad stuff about them. But my bike didnt have a spark when I bought it so maybe keep an eye on electrical.