Thanks all! Cover and head removed and a chipped exhaust valve was discovered. Time to get to work. Looks like I have been running too lean for a while... I will have to adjust carb as well then.
Good to hear that you got it figured out, I suspected a broken exhaust valve since it died in the same manner as Daves. If I recall, his broken valve did no damage to the bore. He just did the overbore because he was using lots of oil anyway. If there's no other damage it should be a relatively easy fix for you!