_it’ll rev itself up a bit high all by itself_
_it keeps going when I let off the throttle_
not a lot to say about it .. something or other about the carb seems to need attention.. idle air flow or choke or float level or something aint right.
Did you measure the new piston and the bore? Normally, the bore should be about 0.002 larger in diameter than the widest part of the piston skirt.. near the very bottom of the skirt, crosswise from the trunnion pin.
Good to hear there's no evidence of seizure. Gap the rings to be sure they are not binding when the engine gets hot.. A ring's end gap when it's slipped into the top of the bore should be near 0.008 inch, at least. Too little gap can make them bind and cause lots of friction (or worse).
And since that's a brand new setup with the new rings, and piston and bore, the heat build up might be explained with normal heavy wear while the parts break-in.. Extra friction could also explain the high idle speed needed to keep it running, although it seems like there is way too much friction somewhere.