Just purchased a new to me 91 CR500. Went for a test drive in the backyard and within 10 seconds the chain popped off. While I was in the shop getting the chain off the bike I noticed that the shifter shaft has a pretty decent amount of in and out play. What could be worn out in the cases that allows this play? Maybe 1/2" or more of in and out play. Makes me kind of nervous with looming interior damage....I've looked up parts for the inside of the cases and a ton of stuff is no longer available. Hope I didn't just buy a expensive paper weight.