I have a 1gm10 which is about a 10hp. It is basically new ie it hasnt been install or turned over yet but it 15years old. I thinking about going to an outboardengine for my 20 ft sailboat. Is that going to be big enough for you?
here is the link to the yanmar industrial engine.
fairly easy to marinise with a radiator,water pump and water sleve on the exhaust.
also possible to have an oil cooler.
just need to fit it inside a box so the cooled air is duckted over the engine.