so at least as far as the westerbeke four-91 is concerned...
i am trying to draw conclusions from the information out there.
I've got 2 separate roads here.
a) westerbeke used the same blocks as, Leyland 154 (also called Leyland 4-25), and Nuffield 4-25.
these parts can be had domestically as Leyland diesel parts or overseas from tractor parts suppliers. In my instance the waterpump differed based on age.
-the actual pump found here-jcl
-the same p# else where-# 12H3203
So, the part started to fuzz and distort it may fit it may not. Which leads me to road (b)
(b) westerbeke in the USA is universal, they use kubota blocks, kubota is a tractor and the engine is D-1101 for a M30. That engine is used in the M5950. And here's that part
These tractor websites will hopefull give some folks some traction.
note:allpartsstore.com makes it easy b/c you can narrow down by part 1st then by mfg/model. so you can see all the potential parts nearly at once.