Regarding the carb screws, they use some sort of locktite on them I think. A little heat from a hand held torch is usually enough, plus the right size of JIS screw driver.
It is odd that there is no damage on the piston, I would almost have bet money on some foreign material getting stuck in the exhaust port, thus scoring both the piston and liner perfectly straight.
Here is what that can look like (on a second hand engine), still runs fine though to my big surprise:
