As someone mentioned already, shaft length may need to be changed, is the engine mount footprint the same? are there clearance issues on the "new" engine (height, width, accessories, access?) Going from Yanmar to Yanmar I'd hope most of these would be minimal, but if changing brands it can amount to a lot of work/money/both.
Sometimes even the angle of the engine beds needs to be altered.
I think you're right to be concerned about 25 years of RWC and its effects on the waterjackets... But realize the cost of the repower is always more than the cost of the engine itself.
Best of luck in your decision.
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)
Last edited by Faster; 02-26-2008 at 07:40 PM.