Depending on the problem, VM diesels were instaled in numerous vehicles, Fords, Jeeps, Rovers, Land Rovers, so you might find specialist help in those communities if you can't find a decent marine mechanic. I have stripped and rebuilt a four cylinder VM car engine, and the Mecruiser/BMW versions are the same but with an extra two cylinders and, of course, raw water cooling. They are not particularly complicated engines, although the separate cylinder heads can cause problems for the unwary.
I had a vm engine on my Vauxhall frontera great engine it just kept going and never let me down - there is a specialist if I remember in chesterfield who has a wealth of knowledge on these engines
jon