You're basically describing the Rotax 912, used on plenty of homebuilt aircraft, microlights and until recently the Piper sport (now back to sportscruiser).
It is part water cooled part aircooled, with 2 radiators which only occasionally leak. Runs upto 6000 rpm, with a fixed ratio gear box so the propeller can make good use of it. 100HP and only 12 litres an hour fuel burn, and runs very happily on the same petrol you put in your car.
If it's a home build, you don't even need to strip it at 2000 hours.
Ultimately, there is more to go wrong with a radiator and gearbox to worry about, and the high revs over time will take the toll, but with a lower outlay to buy the engine, lower fuel costs and overall lower maintenance it seems to be the way a lot of manufacturers are going.