That is not piston wear. Your picture is what gets called 'piston failure'. When aluminum (piston top) melts and flows out the exhaust port there are a variety of faces painted on piston faces. You say one piston (PTO) side looks good and this one smiles from the MAG side, I wonder what it could be. See I really do not know yet, but lets investigate.
1. Did you change your crank case seals?
2. Are the proper clean jets in your carburetor with air box?
Guess, guess, guess. Pull the motor it takes about 30 minutes. Disassemble (so you can put in new crank seal) and report back with a picture of the Mag side seal. I suspect you will find your answer.