The OS 50 is famous to injest a rear bearing at about 10 hours. Here is your fix. Get the cheap rc-bearings . The head is fine and the high spots can be sanded off with head gasket removed. The piston ring grove is probably pinched and a new piston is in order and use the old ring. Lightly sand the liner to break in the ring again. Flush the case out good and put back togeather. If the ring is loose in the piston the top of piston can be dressed up a bit and make sure the nicked edge of the top of piston is slightly filed or dressed with sandpaper. Don't sand head or piston to get all the deep pitts out. Just enough to knock down the protruding high spots. The engine will run again.