Looks like some blow by in the last pic? Or a shadow?
I would rebuild it. If they are cast pistons, although they are fine and you can run a tighter wall clearance on them, they don’t like to go past service intervals or put up with as much abuse as forged pistons, last thing you want is a cracked skirt and wrecked motor.
If you don’t know what’s in it or how many hours it has done I would rebuild the top end and inspect everything else. At least then you know what you have and can have confidence in your machine.
While it’s open clean up that exhaust port, If you want to eliminate or at least greatly reduce that carbon buildup polish it and use a good ester synthetic oil