Small Engines (Lawn Mowers, etc.): Small Water Cooled Engine, briggs and stratton, stroke engine

Question
I'm in need of a small (10+ HP) 4 stroke engine to run a generator, but it needs to be water cooled. First, does such an engine already exist (such as a boat engine, but most are 2 cycle, or a garden tractor engine, but most are air cooled.) Second, would it be difficult to mount a water jacket over and around the head and cylinder of an existing air cooled engine? (Welding would likely warp the head, and any glue or epoxy might crack from vibration.) There are also maintenance issues for accessing the spark plug, and maybe even removing the head later on. Whatever sealing method would need to withstand 20 (min) to 50+ (ideally) PSI. Out of curiosity, how much heat would escape from the head if a a water jacket was installed around just the cylinder alone? Any thoughts or suggestions would be very much appreciated. Thanks!

Answer
Kawasaki, Kohler, and Briggs and Stratton all have some type of water cooled engine. Most are larger than 10 horsepower, closer to 26 and up. I don't have any idea about modifying an existing air cooled engine, sounds like something that would be impossible to rig for longtime reliability.