sure Rotax is king at the moment, but thats not to say that there arent any other good reliable options out there. The HKS i will never purchase, they are a ripp off in the automotive game and for a half and engine with a turbo on, they are still a ripp off. Their automotive products are junk, and its the same company that manufactures the aviation products, i personally have had experence with their automotive products and all our customer who share the experience will advise stay far away!
Rotec radial engines (sure depening what plane you fly) are also awesome engines, but expensive. I like the idea of the pricing for the verner option, BUT this must be reliable. There are referrences who use the engines in the USA, who i would contact first, but at the end of the day think about it from a business point of view - a company would not invest more and more money into developing additional engines (verner is now on 6 kinds of engines running, in the market) when you not selling the first one.
So far from what i could find, no failures, poor service related issues or anything else -- maybe not enough of them in the market, who knows. Id be keen to try one, then again i am normally the first to try most things (just look at my brakes and 914 project .... )
BMW started out making aircraft engines, so i would also consider that, but the 1200cc GS engine is VERY rough, especially when coming onto throttle, I know, i own a 1200GAS adventure, and when you rev the engine, the whole bike moves from the torque -- imagine this in a light plane like a cheetah.... mmmhh
