Why buy someone else's problems?
The start of our project is mentioned here:
https://www.ericsonyachts.org/infoexchange/showthread.php?15453-Universal-M25XP-for-sale-used
And, I have several Blog entries on this site about the changeover.
If you decide to put in a new engine, you will like it so much that you will want to keep your boat another ten years!
Not trying to be 'flip' but please do all the $ math for a rebuild and re-install.
New wiring harness, probably new panel, replace exhaust, new torque plate, very likely a new transmission, tear down and rebuild the short block, seals, etc, and some more etc.
I would guess that you will wind up with, to a shopper somewhere down the road, a still-OLD engine, and be out the $$ for 2/3 of a new one.
I had one offer for my old running M25XP, and that was for 1K, plus some shipping. Really not a good deal for me or the buyer, unless he also owned a machine shop. (?)
You have enough hours (twice what I had) on your existing engine that it now owes you nothing more. It has served you well....
Further: I have helped a friend with some little projects in the engine compartment of his 32-3. There is darned little room to spare in there at all. It's a very tight space.
The smaller dimensions of the Betamarine 25 would help quite a bit.