First, if the MS -38 is complete, with a decent gas
tank, and has the spark plug cover at an affordable
price, you should grab it. It is way more desirable
than your motor. Just the spark plug cover, in good
condition, has more value than your motor complete.
Same for the gas tank. I have no experience with
the power head, however I would suggest you change
your approach to repair as needed rather than taking
something apart before you find a need to. I do have
some experience with the lower unit though from
having other models. A piston works off a cam on
the prop shaft. You should clean out any grease
remaining in the gear case and inspect the check
valves to insure they are clean and operate freely
before refilling. Hopefully your motor was put up
without any water remaining in the gear case.
Tubs
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.