I rebuilt a 1993 KX 500. It was a fun project, but got a little pricey, but it was well worth it in the end.
When I tore the motor down, I found out that the chain skipped and hit the case where the clutch pull is located and cracked it. I was able to obtain new cases from a 2004 model. After all the bearings and seals were purchased for the bottom end, I had a friend who used to wrench for Team Green assemble it for me. He modified the third gear for me, ( I still don't know what that means) and put it all together for me. I sent the cylinder out to Mellinium to be replated. While assembling the motor, my buddy did a modification to the KIPS valve system. He also rebuilt my forks and shock for me as well.
The bike is an awesome ride for being 21 years old. It starts up easily and runs perfect. A few months ago, I acquired a 2002 rolling chassis and put the motor in it. Everything is the same, but now its a 2002 KX 500.