One of the original spec numbers was 64509. The shortblock is still available. The 64638 and 76607 specs were both built for Cub Cadet although minor parts may have to be used from the orignal engine. If you use the CH620-0017 or the CH22S-66532-SD engine, the mounting is the same and the shaft diameter is the same. The shaft length is 4" but could be cut down as the shaft is not hardened steel and is drilled and tapped 2 deep. You would probably have to use your alternator and regulator and throttle/ choke controls off the old engine. You would also have to bypass the keyswitch on the engine. Parts would be interchangeable.