My first questions would be why is the Yanmar inoperative? Fatal problem? May well be easier to fix what's there in the long run..
If you are planning/able to reuse the existing gearbox than there are no concerns about prop pitch and gear rations as none of that should change unless the engines operate in much different rpm ranges... although the increase in power might suggest a slightly different or re-pitched prop
Nowadays engineering drawings are readily available.. google these motors, find some literature and compare the overall dimensions, footprint and mount locations to get an idea of whether or not you're going to need to seriously change the beds and mounts.
It's always a challenge to repower with a totally different motor unless it's a designed drop-in replacement. Finding the adaptor to connect to your existing gear may be as big a challenge as any of it.. you'll need to know the specifics of the gearbox too.
".. 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)
1984 Fast/Nicholson 345 "FastForward"