If you're looking for a direct drop in look at the Beta Marine
engines. They make a really nice marine version of a Kubota engine that are very reliable and smooth. The plus is they've set up the engine mounts to fit an A4 almost exactly!
Are you planning on doing some long range cruising? If so definitely go the diesel route
. However if you'll just be doing some coastal cruising or weekend use for the boat the Atomic Four is a great engine. Many people are afraid of the gasoline aspect, and it does deserve serious respect for safety
. However I believe it is still the most widely produced marine engine in existence. It was made for right about 40-50years. There are many of them still in regular use. If you were to look at a safety
report that accounted for the per capita or percentage of the engines in use I'd be the A4 would rate quiet well. I just sold my old boat with an A4, the engine was from 1959, the boat a 1961. The engine had new bearings/ rebuild
in 2001, and was running like a top when sold. I've heard in general you'll spend about $9-14k re-powering with a diesel. That will of course depend how much you do yourself and how many parts
like the fuel tank
you can reuse. Plan on replacing the prop, prop shaft, coupling, engine panel, controls, muffler
, and cutlass bearing as well, because they will likely be the wrong size. You can still get a very nice rebuilt A4 from Moyer Marine and get out of the re-power for about $3-5k.
In short our A4 was great it: required very little maintenance
, gave .75gph burn at 6.4knts, smooth, and reliable. It did make me very conscious if I left the fuel valve open or not, and of running the bilge
blower and checking the engine compartment before starting.
Good luck either way.