Charles, the Universal diesel engine in our boats is a marinized Kubota tractor engine, and non-marine Kubota components are typically available at (so they say) reasonable cost. Are you a member (small 'm') of this Forum only, or are you a paid-up Member (capital 'M') of our association, the C36IA??? If you're a paid Member, then you have full access to our incredible library of technical articles, accessable from the drop down menu on the C36IA home page. Our past Mk I technical guru wrote a fine article on removing and overhauling the engine you're considering. Have a look there.

"Marinizing" is essentially the conversion of the cooling system. In both applications, the internal high-pressure cooling is the same closed anti-freeze based system. The only thing that changes is how the heat removed from the guts of the engine by this internal coolant is itself dissipated: either by airflow through a radiator (the tractor), or by water flow through what we call a heat exchanger (the boat).

Beta Marine is a popular repower option as they will mostly fit without too much fuss - there is an article in the Tech section about repowering with a beta.  If your adventurous, you could get new head, pistons, cylinders , gaskets for <$1400 - and rebuild it your self with "tractor" parts.   Search ebay with "kubota D950".   Depends on why you lost compression - head gasket, valve - or rings.  Head gaskets are cheap, but by now you would probably have water damage,

I believe the M25XP is a D950 engine.
Another great resource is Stu's engine wiki over on the c34.org site.  It has tons of info on many systems that are common between the models.

good luck -

Les