Re-reading your post, methinks this mechanic must be a pretty special guy being able to measure the piston play and cilinder roundness with head and cilinder installed. Did he measure exhaust temperature with a thermometer and I would be interested to know what figure it should be. You sure you got the right establishment for the job?
Leakdown test; of course this is something of an indication.
But, following is also true: the only indication of piston wear, within limits, is oil consumption, and not engine ' behaviour' as to torque or output. Within reasonable limits, that is.
So if you don't see the telltale smoke from the exhaust, look for the problem somewhere else.
Exchange CDI's, sparkplugs, check carbs, wiring, etc.
Good luck!
Auke
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