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'99 yz125 jetting for high altitude

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I'm new to the site and the sport so any advice will be greatly appreciated. I bought a 99' yz 125 at a yard sale for a few hundred bucks, and my buddy who talked me into it was kind enough to rebuild the motor (new bearings, crank, basically brand new again) A brand new procircuit spark arrestor for a yz250 came with it which he refabbed to fit my bike. Other than the pipe, all is stock. Yes, total low budget project but its my first bike, was only ridden 1 season, and basically brand new. I got exactly 3 miles until melting a hole in the piston (already replaced) and dont want it to happen again. I was told its way to lean and needs to be rejetted. Here's the kicker, I live at 8,650 ft and the trails i want to ride ascend and descend between 8,500 and 12,000ft. The shops i've asked are pretty tight lipped without handing over a bunch of greenbacks and having them do it for our base elevation. Any advice (other than getting a bigger bike or 4stroke) on what jets to install for altitude would be great. Thanks a lot!!!!

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