I have four of these engines in my garage. There is zero chance they would make good aircraft engines. They make their power at 9000 RPM and above. If you could stand the noise–which you couldn’t–they wouldn’t last long at the 70 percent duty cycle of aircraft engines.
Don’t forget Rotax tried this with a sophisticated V-6 14 years ago. They abandoned it as untenable. But it could have worked with more development dollars and time. If sorted out, it would have been a lovely engine.
And all the people here complaining about 100LL and pining for new technology wouldn’t have bought it. Which Rotax fully understood.