Ive been this route before. I bought an 87 XR600 for $400, The guy told me that cam chain had worn out and slipped a tooth, but he didnt think the valves were bent. He also said it had a fresh top end. I thought, OK even if the valves are bent, that wont be too bad. I pull it apart, and sure enough the valves are bent. Upon closer inspection, it has a big bore kit that was put in in 1990 (XRs only conveniently put the date and size and all that on the piston) and it was tottally shot. So I decided to go throught the motor completely. Next thing I found was a few teeth missing in the tranny. just like you I decided to fix it, and I still dont know why. About $1000 later, I had a pretty much brand new engine and transmission and everything was good for about 6 months. Then about the only part that wasnt replaced, the con rod, decided to give out and took the piston, cases, cylinder, valves, just about everything with it. At this point I just gave up and sold it to a scrap dealer for $300. I took a loss, but once I got into it, there was no going back. My suggestion for you would be to just keep it and run it, cause your going to be losing money either way, and if you do buy another bike the same age, your taking a gamble on it too. Whats everybody else think?