My OS four stroke engines have excellent to very good cold compression. Many people say that ringed engines have poor compression when turned over by hand. I don't believe that to be true for a good engine in good condition.

When turning the engine over listen to the exhaust pipes for leakage. This would indicate exhaust valve leak. Listen to the intake for intake valve leakage. Verify the tappet clearance is correct. If there is no leakage of the valves the only thing left is the ring. It could be stuck with old castor or just worn with a large ring gap. The piston could also be worn not fitting the bore properly, thus not allowing the ring to seal well. Also the cylinder could be scored. Inspect the cylinder visually.

A little leakage won't really reduce performance. If you can't start it by hand I would would do repairs. On the other hand you could just run it until running becomes an issue.