You might try these guys... the A4 gurus: Moyer Marine Atomic 4 Engine Rebuilding and Parts
Adapting to the existing mounts should be possible, esp if the beds are wider rather than narrower. I'm pretty sure an A4 was an original option with these boats, so something should be available.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)