Interesting conundrums with which I am very familiar. Our MD3B started easy, ran well, made a racket, leaked oil, left a filmy trail the EPA, Coast Guard, and Fisheries could trace to my door. Time for action. Read all about it in my thread:
http://www.albinowners.net/aog/viewtopi ... 17C#p59640
The reason I chose to work on the 17C vs. the 3B is that the 17C has replaceable liners, where the 3B cylinders are of a piece with the water jacket...more machine work may be involved. Not impossible to be sure, but more fooling.
There are a lot of issues involved with installing a different engine...mounts, shaft, controls, prop, engine bed mods, exhaust improvements, electrical (maybe)...the list goes on. None of these are cheap to accomplish and add directly to the cost of a different engine. Even a FREE different engine.
The main thing with the 3B is how much do you know about its condition, hours, maintenance, oil pressure, FWC or seawater cooled... Given its age if it is still running as the original seawater (No heat exchanger) cooled, either it has very few hours, was meticulously maintained or run only in fresh water. If it has an (aftermarket, the only way they came back then) HX, then that is a positive.
Lots to ponder. Think it through. Go rob a bank...kidding and keep us posted.
ps.. After all I have been through, maybe, just maybe a brand new in the box modern engine would be nice. The trouble is, is that all this experience with working on this engine and this boat, only NOW do I have the knowledge of how to do an engine swap.
And I'd still be at least 10-15 Grand poorer.
If buying used, I'd suggest getting a documented runner and install as is. Good luck finding that engine. Oh, you'll find a seller, guaranteed. Remember, you will only be putting a couple hundred hours/year on the boat, at best. A top end rebuild on the 3B is very doable and not difficult. May be good enough if oil pressure is good.