there is another possibility- a broken bearing. if the rear bearing has a metal retainer, when the retainer breaks, the metal fragments get ingested- they blow up the bypass and into the cylinder, where they bounce around and ding up the piston crown and the combustion chamber. if they get lodged in the port, they will damage the side of the piston and the ring and maybe the liner too. i have never seen detonation cause this kind of damage, though i suppose it is possible, but i have had damage like this caused by bearing failure several times! be sure to check your bearing- pieces of the retainer can break off and you cannot really tell from outside the engine, it still turns over smoothly! if your retainer is broken, you have your culprit, if the bearing looks new, it was probably replaced by the previous owner. if you need to replace your bearing, consider one with a phenolic retainer, they do not have this failure mode, but they are more expensive. one other thing- on a ringed engine, the squishing of the piston caused by the bearing schrapnel can cause the ring groove on the piston to be squeezed shut, clamping the ring tight in the groove and then the ring cannot move properly to seal against the cylinder. since your ring broke, be sure to check the piston to be sure the new ring has clearance to the ring groove.