Re: ISLANDER 29 repower with YANMAR 2GM?
Try an google image search for both engines.. you're likely to get drawings from manuals that have been published on line... and then compare the footprints to give you an idea.
Years back we removed an Atomic2 (yes, 2 - not a typo) and dropped in a new Volvo 2001.. the mount spacing was almost 4 inches narrower than the existing beds. We through-bolted (to the original beds) aluminum angle iron to narrow up and match the mounting angle difference.
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)