Silvaire you are barking up the wrong tree.
The Rotax actully makes MORE power. Well to be precise more usuable power. Having run the same airframe with an O-200 and 912S this is definitely true.
At 100% the engines do indeed produce the same power. However apart from in a dive the O-200 cannot make the 2750rpm needed to produce 100hp. Of much more importance is the 75% figure - which from my Continental manual was 2475 - an rpm figure which again it could not make in level flight.
So the most it could manage was about 55% static and 70% in level flight - so 55hp to get off the ground and 70 odd in level fight.
In very, very, stark contrast the 912S can turn up to 85% in level flight - giving significantly higher speeds. Static it produces close to 70% - giving much better takeoff performance.
And whilst it is producing this extra performace it averages 4 lt/hr less consumption!!!!