To answer your question about the starter motor, I think you'll find the isolated ground doesn't make a difference. If my memory serves me correctly (at least on the Perkins) the ignition switch operates a relay to temporarily ground the engine so the starter works rather than having a positive and negative lead to the starter (easy enough to check by looking at it). Another relay does the same for the glow plugs and another for the stop solenoid.
Be a bit careful crossing over Perkins and Volvo parts. I believe one of the differences between the Perkins and the Volvo is that the wiring is quite different and suspect the same goes for the seawater system. As mentioned already the same basic engine applies across Perkins M50 or M60 (different top RPM) equivalent to the non turbo Volvo MD22 and the Perkins M80T is equivalent to the turbo Volvo TMD22. Filters, timing belt parts, gaskets etc should be fine.
I would guess the starter is a standard Lucas one and would be available from an autoelectrician but haven't tried it.
