[MOTORHEAD]

50/50 mix of what?? Is the engine freash water cooled, and if so,
has it been such all it's life? What year is the boat? How many hours are one the engine? The engine would have to be torn down to ck for cracks. You need all the information you can get so you can make an informed decision. Unless the owner lets you take the engine and disassemble and check it before you pay him
(yeah! that'll happen!!!) or he gives you a WRITTEN guarantee
that the block, heads, and crank are not cracked and are rebuildable, a marine engine of this vintage is a "pig in a poke".
What is his asking price? If it's dirt cheap, and the thing has always been fresh water cooled, you might roll the dice and buy it, but at least do a compression test. Is iy in the boat and running now? Get a mechanic to check it out for you, with you being present. Does he want big money? Do the above, plus get a guaranty, maybe do an escrow with the money.