If it is the exhaust valve leaking, you should hear it in the exhaust pipe when you turn the engine over on compression. If it is the intake valve, you should hear it in the carb. If you don't hear it in either one, it is probably the ring. Add a little oil through the exhaust port. If compression comes back it is most likely the ring. If not, you probably have a glow plug leaking. I have seen bad pushrods on Saito's that would give good compression one time and then hold the valve open slightly on the next time. You can tell this by opening up the valve clearance to maybe .030" and see if you then have compression.