My 1976 Yamaha RD400C had a a small section/portion of the intake airbox that you could remove for high altitude riding. Removing this part returned the performance back to reasonable levels at ~ 8 thousand feet and above (it was a real dog at these altitudes unless you removed this part). Maybe you could try running a less restrictive air filter or airfilter cover at high altitudes... Anyway, I recently ordered a couple of stock connecting rod small end bushings and received needle type bearings by mistake. Today I checked one of these needle bearings out only to find that there is no way I can run them. There's way way too much end-play or "side to side" clearance between the needle bearing/rod end and the piston pin bosses. Oh well, I've got another upgraded bushing on hand that I should be able to press in and ream as needed (I wanted to try a "practice" run on the bushing replacement procedure with a cheap stock bushing first).