driveshafts are interchangable from 1956-1976. Water tubes, once introduced, were interchangable and you should make sure those O-rings are new on them too (for the driveshaft water tube).

Pretty much other than the powerhead most parts from the 18-25hp family are interchangable. Beware of the gearcases though as in the late 60’s they started changing the gearsets to make them more durable. Some gearsets are splined in opposite directions, some use a roller bearing for the forward gear, some do not. Some have a spring loaded clutch dog detent/retainer, some do not.

The most coveted are the 3-lug gearsets, which are fairly rare. If you find one of those, hold onto it!