I've been down this road recently for under 50 hp sailing auxiliaries
generally available for purchase in the US Here's what I found:

"Marinization" is the process of adding (to a basic industrial diesel
engine)

Raw water pump and hoses
Heat exchanger / exhaust elbow
Air intake filter
Basic electrics (alternator, wiring harness, engine low/high alarms
etc)
Bell housing, ie. everything aft of the flywheel
Mounting brackets and vibration dampers
Accessories (instrument panel, oil sump pump and the like)

The Kubota blocks are marinzed by:

Beta
Universal
Phasor

The Mitsubishi blocks are marinized by:

Westerbeke
Vetus

The Perkins blocks are marinized by:

Volvo

There are a few 'independent engine, someone else's transmission'

Volkswagen Marine (very cool, IMHO but smallest is 40 hp) see:
http://www.vw-m.de/index.php?L=1&id=10
Lombardini

There's one completely independent (engine and transmission):

Yanmar

It's interesting to note that Westebeke markets both the Mitsubishi
(under their banner) and the Kubota (under the Universal banner). The
Mitsubishi/Westerbeke is thir 'flagship' and is about 20% more
expensive than the Universal. One might decuce that either the
Mitsubishi is a 'better' engine than the Kubota, or that Westerbeke
takes more "care' in the marinization of the Mitsu...

I've seen the Phasor at various boat shows and was impressed by their
marinization - stainless steel exhaust riser, silicon hoses, decent
sized alternator as standard equipment. For prices, visit a major
boat show where all of these guys exhibit - play one off the other

MW
Los Angeles