Aah, the plug is black so that would indicate its a bit rich, a much better situation than lean.
You should strip and clean the carb and make a note of the numbers on the jets, needle carb slide and clip position just in case you find a manual somewhere to reference to.
I suspect that the problem is mechanical failure and not caused by carburation, has the bore been damaged? does it need a re-bore?.
Check the bore wear before you order a new piston because it might have a worn bore and you would have wasted your money.