If the inside of the cylinder looks ok with no deep scratches its most likely the head gasket that went... if you see a dark area on part of the gasket this is where it was leaking out... a new head gasket and lapping the head on a piece of sandpaper over a flat surface like a sheet of glass until it sands the whole sealing surface evenly and any dark areas are gone then its ready to reinstall the head with a new gasket, torque 120 to 140 inch pounds in an X pattern and it should be ok...
If you see scratches in your cylinder post pics so we can see if it needs a new jug, piston, and rings... this can happen from overheating or inadequate air filtration and riding in a dusty environment.