as far as the cooling issue, i would think that maybe with some sort of vented reservoir, like an overflow tank, you could probably solve any sort of recirculation or pressue issue.
my other thought on this would be.... what would the intended use be? these types of engines are generally of the type to make power at high revolutions. most agricultural use machinery is more of the slow but steady theme, hence the popularity of the diesel engine.
years back my uncle made a low budget version of a cushman out of an old 3 wheel harley golf cart. had a light gage steel dump body with an electric/hydraulic dump. the dump worked ok, but the motivation system was the issue. the first powerplant was an ancient ski-doo free air snowmobile engine. this didn't work too well - too erratic for putting around. the second attempt was a honda atc engine. this was a little better, but proved to be too slow with the atc transmission and golf cart rear end. in the end, the only thing that either of these excelled in was making a lot of noise.