I helped a buddy rebuild his Westerbeke .. we found that it was a Mitsubishi tractor motor and once we figured out which one, were able to source parts from Mitsu.. We also found that Westerbeke has some very strict agreements with the folks who supply their engines (they use a couple of different suppliers), so that no parts can (knowingly) be sold through the engine supplier; the parts from Westerbeke are 50% to 200% more. We found this when we tried to buy from one Mitsu guy and made the mistake of telling him it was for a Westerbeke marinized engine.. As RitSim notes above, with a little detective work, you can usually find what ya need for the basic engine.. Good Luck.
EDIT: we found the engine to be a Mitsu when we saw the little three bladed propeller logo inside this piston. (picture does not show it)
