You didn't say which model of MD6 you have, but I expect the Volve Penta store will have what you want. I've never used them and don't know anything about them, so you're on your own there.
Anyway, take a look at this page
and choose the appropriate "Cooling System, Induction- And Exhaust Manifold" hyperlink for your model under Cooling System - 7742080
If you find a better place to buy, be sure to share it here. There are a great many Islanders with Volvo Pentas.