I hate that BS mentality of some of the auto parts stores. In part those people are just being a bit lazy as their computer systems are set up ONLY for finding the right part for your CAR, like many of the auto parts websites.
When I go to get spark plugs for my Atomic 4 engine I am often asked the vehicle make and model. I just tell them it is a tractor engine which seems to shut them up. I already know the various mfr part #'s I want for the sparks (Champion RJC 12 etc) so what does it flocking matter what vehicle?
I looked for a cross reference for Westerbeke/Mitsubishi engines and stumbled onto this post: Yahoo! Groups
This makes me think that the Mitsu block # may appear somewhere on the head or block. Once you find a block # I'd then try the store linked to in the post: Parts Supply Corporation for Industrial and Construction Engine Rebuild Parts. Diesel, Marine, and Heavy Industry applications. Engine Overhaul Kits, Pistons, RIngs, Crankshafts, Gasket Kits, Cylinder Head Rebuild Kits, everything you need to put it