Replacemet engine

Hello to all
I have a 1990 runabout with a 4.3 merc v6,alpha one, the boat is solid,the out drive was rebuild last year with o hrs on it. the problem im having is the v6 has water in the oil,im in the processs of tearing it down this weekend, weather permitting to see if is cracked or a gasket.the question i have is in reference to a replacement engine,if i need one. i would like to know if a v8 or a 4 cylinder would fit with no alterations.if been told yes/no,does it have to be a merc engine? can it be a ford?ive been told that the year of the engine has alot to do with it.any help will be appreciated.
Hector