It has the right symptoms for having been underlubricated when you worked it hard, and having scored the piston as a result. There are other possibilities, but it might be a good idea to remove the muffler and look at the piston through the exhaust port. If you see significant scoring on it, that is likely to be the problem. You would then need to inspect the bore and see whether it just needs a new piston and rings, or perhaps a bore hone as well.
If the piston looks good through the exhaust port, please post some pictures and more details, and we can talk about other possibilities.
