A 3-20 hp vertical shaft lawnmower engine will works too.Replace the bowl carburetor with a marine diaphragm carb.Keep in mind that whatever prop you use may have to be a left hand prop.I helped design a blower like this for a friend.You may have to have a prop shop bend a prop to be a left hand.Or cast your own prop.Or design it with a gearbox that reverses the prop.Also you have to have a way to attach the prop to a shaft.Most prop shafts are splined so you can use the stainless steel shaft off a outboard engine.And extend the shaft using some kind of a coupler that wont vibrate loose.You will need waterproof sealed bearings to support the shaft in the tube.Or just use brass or stainless steel bushings.You can use aluminum or PVC for the tube.You can have slots or cut outs for the water intake.Or extend the tube way below the engine with support brackets that are welded or bolted so they dont fall apart.Its all trial and error when building something like this.Honda lawnmower engines are the best to use.They start easy and are quiet.Some even have electric start.