If its an Atomic-4, the only real problem that occurs is the block rusts through; but, can prevented by installing a fresh water heat exchanger. If youre buying a boat with an A4 simply remove one of the exhaust manifold bolts and go into the hole with a probe to count the number of threads remaining - 4 or more means that there is sufficient iron in the block (means it can be rebuilt if necessary). Atomic-4s rarely ever "wear" out, can be ''modernized'' with electronic ignition, electric fuel pumps, PCV valve, oil filters, etc. Parts are still easy to get. there is a Atomic4 discussion group here on Sailnet. Moyer Marine, Harrisburg PA publishes an excellent Maintenance and Rebuild manual ..... needed as most garden variety ''marine'' mechanics dont have the knowledge to maintain or rebuild one and usually replace a LOT of parts until it works.
If its a Palmer engine, parts will be very hard to get.
Overall the Atomic 4 is vastly smoother running and almost as economical to run as a diesel .... the advantage is: you yourself CAN fix it with a little knowledge and a good manual.