Zenoahs are very good engines, for less of a cost, the CY engine is the engine of my choice. The CY is basically the same as the zenoah, except the CY out of the box has more HP. The ports are cut larger at the factory, it is easily upgraded.
[link=http://oneillbrothersracing.com/rccar/motors.htm]Oneill Brothers Racing[/link] all of the engines on this page will work for a airboat. Lester Burch with [link=http://kirkwoodrc.com/2401.html]Mudmasher Airboats[/link] has or can make the prop hub for any of the engines shown on the above link for Oneill Brothers.
You can get a engine with the clutch and the prop hub will also mount right on it. Clutch on a airboat is ok but not needed.