It sounds like a detonation seizure. I did the same thing when I first had mine. Too small mains and low octane fuel will do this. Advance will also cause it but I don't know how much the HRC box pushes the timing. If it was oil, you would most likely get a 4 corner seizure and both cyclinders would be done in. It's no big deal, anyway.
If you are using the PJs, start around 170 and work down. You'll probably end up about 150-160. At least that was my experience. CHOP, chop, chop, or go on the dyno. Pump gas will work fine for plug chops, but be consistent and use the same gas every time.
Buy a new piston and send the cylinder and piston to the Nikasil plater for a replate. Be sure to tell the plater to reinforce the exhaust port bridge and refit the power valve. Eric Gorr does a great job with this. He will also heat treat the cylinder to relieve welding stress prior to plating. You can clean the cylinder with hydrochloric acid. Maybe it can be saved.