At 20 HP, you're in the "Atomic 4 Replacemnt" group, which is well
populated.
Several manufacturers offer 'Atomic 4 Replacement' diesel engines. Many
are based on the same Kubota 3 cylinder block, the only difference
being the 'marinization' of the engine.
Yanmar has a similar size / weight engine which is not based on Kubota
- it's a Yanmar (!), the Vetus is a Mitsubishi, and I believe the
Volvo is a Perkins

Here's the list:

Universal M3-20B
Phasor Marine P3-20-SM
Beta Marine BD722

Yanmar 3YM20
Westerbeke 20B2 or 30B3
Vetus M 3.09
Volvo D1-20

I saw this engine at the Annapolis show, and it has some nice features

Lombardini LDW 702 M


If weight is a concern, the first 3 (the Kubota derivatives) and the
Lombardini have a
'dry' weight of around 230 lbs. The last four are closer to 290 lbs.

The Volvo is slower turning (3200 RPM vs 3600 for the rest) *and* comes
standard with a whopping 115 Amp alternator.

The cost of the basic engine + standard gear reduction (less high-end
meter/light panel or other options) is *around* 5.8 - 8.5K$US. Often
the best deals are had at boat shows, so if you're looking to save $,
enquire at a major show. This price does *not* include installation;
you get the engine on a wooden pallet.


MW