If it locks up intermittantly Id guess maybe a couple of things: Piston broken/cracked, and catching on the edges of the ports, would be hellish noisy tho when it did spin over. Possible bottom end seizure, maybe bearings in main bearings misplaced, or possibly a damaged or cracked gear in the primary drive. One other thought, where has all the coolant gone?? if the base gasket or head gasket has failed, all the pretty green coolant may well be sitting in the bottom of the engine, causing the primary compression to be huge, and stopping the motor spin over. Quit guessing and pull the head and barrel off, its maybe 2 hours work, and will reveal the extent of the problems.