The hondas seem to be the best of the mini four strokes burning pure gasoline out there at this time- I think some of the pre-mix burning four stroke designs may have superior power- but are more expensive and harder to locate.
There are quite a few folks using them on planes. They are fairly easy to make a prop adapter for- the crank thread on the GX35 is 8mm X 1.25 pitch. The oil isn't in a true sump- it is more like a "mixing chamber" where it is constantly whipped into a mist by a two bladed dipper on the crankshaft- then the mist is forced to flow to the critical parts. There are valved drainbacks that return the oil to the chamber. The system works very good at any orientation. The engines are high quality and very user friendly and easy to work on. Just do a search in here on "honda GX" and you can see some information on them.

I have experience with the Honda 50cc also- I use them on scooters. Very strong engine- alas, though, it is a conventional oil sump design
similar to a briggs and stratton type oiling system- it will not tolerate running upside down. The 50cc engines are heavy also- in my opinion, too heavy for RC use- except maybe for some vehicle where weight isn't a factor. The crankcase is pretty large- and can't be effectively cut down to a smaller size. I have been waiting for someone to try one on a plane- but I think when folks get one in their hands- they come to the same conclusions I have about the weight.

If Honda would take the 50cc and make it with the same design as the 25 & 35- then we would have an excellent engine to play with.