Do a compression check, they tended to get hot and crack the block if they lost the water pump. These also had issues with the exhaust manifold perforating when run a lot in salt water.
If I were gonna work on one of these, I'd be tempted to pull the exhaust manifold to inspect the pistons, and renew gaskets.
Also pull the cyl block water passage cover (back of the block), as these tend to leak around the spark plug holes. Use heat if any of the bolts don't want to go; if you force them, they'll break. Check the tops of the cylinders for cracks (especially #1). If the block has a big crack in it, it's non-repairable.
They're a light motor but don't have a ton of low-end torque. They should be propped to spin up to the high-end of their rpm range at WOT. IIRC, the Triples had an "overdrive" set of gears in the lower unit and might run even better with a 50hp lower unit, which had 2:1 gears.
Be aware there are differences in mounting bolt spacing, shift shaft styles, etc in the mid-70's 50hp & 3-cyl lower units, so you have to make sure whatever you get is gonna bolt up properly to your 65hp mid-section.
Measure the hole spacing for the top two studs at the fwd end of the L/U, as I recall these had different spacing on different years. HTH..........ed