Check the cylinder for seizure. Look at the exhaust port for carbon scoring.
Dirty carburetor jets or fuel/air mix issues. Inspect the main jet.
Check the plug gap and fouling. Replace if porcelain is cracked.
Check crank bearings and flywheel alignment.
Heavily coated in black carbon. Likely cause of high compression and pre-ignition.
The engine is running perfectly. Your technical precision saved the day.