You could be having problems with the powervalve gumming up, which causes the teeth on the valve to strip right off. 89-94 KDX's are notorious for this, particularly the left hand valve. This will hamper power (expecially low-end power) pretty quickly if not fixed. You can check by taking off the pipe and looking up into the ports. On the right side of the cylinder is a plastic cover (3 bolts) and under that is a rod that goes into the cylinder (with another rod attached to that). If you pull on that, you should see BOTH ports beside the main exhuast port open up. If not then the teeth are probably stripped on the valves (aluminum). Good luck.
PS-If the walls are scratched on the cylinder and it is factory plated (IE not sleeved) then make sure you get it replated and NOT honed. Honing will only destroy the plating inside the cylinder. I had this done at Langcourt(.com), they do excellent work for relatively cheap. :thumb:
PPS-If the cylinder checks out and you need a new piston/rings, go with Pro-X piston kit. Its the same as OEM; Wiseco won't last as long because of more piston slap.