There are a few options for this, and yes i think the gyro world leads the way in this field, there is a guy in the US doing conversions on the yamaha snow sled engines, 80hp and 110hp i think and there is another type of engine,,,,the Weber???? it might be, somewhere in the 100hp range, this is being tried on the factory type gyro in Europe, of course there is heaps of ea81, ej 22, 25, and some are now trying the 6 cyc ej30 subaru engine, but for around the 25lt + per hour, it only the saving on the engine really.
There are others also, this is just what i can think of off the top of my head.
Actually the honda ST 1100 engine is another engine very possible of use, as the gearbox unbolts off the motor, good power ect. The seadoo jetskis use a, get this, "250hp" super charged rotax, but about the only way of getting one of these is to buy a ski, they wont sell the motor separetely,,, think some one told them what they be used for :)