Of all the things to spend money on in a 1971 boat, a diesel refit is not one of them unless the rest of the boat is pristine. There is simply no need, when - at worst - a Moyer rebuilt A4 that is better than new can be dropped into the boat in, literally, less than a day, if you want to, for under $5k.
And chances are very high that the A4 in there now is not permanently seized. Once you get her running you may be amazed at how straightforward routine maintenance is. And if the concept of learning a little about gas engines is inherently unpleasant, you are not going to like the rest of nearly 50-year-old boat ownership either.
The world has changed since the early 1990s, when the poor A4 was an abandoned engine for which the replacement head gasket was - literally - a piece of cardboard, and for which there was no internet to accumulate owners’ wisdom and expertise. No wonder people replaced their A4s in droves back then. It is incredible what Moyer and his son-in-law have done to turn it into a bona fide good old engine.