Re: engine change out
No question the re&re will go much more smoothly with an engine with the same mount patterns, footprint, overall size, and mounting angle. Also you're more likely to maintain your gear ratios by sticking with yanmar. Unless there's a significant price difference that makes it worthwhile I'd probably stick with the direct replacement.
I've 'been there', hacking out motor beds to adapt a Volvo into a boat that started out with an Atomic.. it worked in the end, but was a lot of work and bother.
".. 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"