Rotornut,
the Mac engine are longer used as they have been. The actual engines offer is far better and reliable.
VW engines work very fine but they tend to disapear (heavy), in favour of the small rotax 2 stroke engines.
For example: take a rotax 670 skidoo engine, it is 105 HP for about 50-60 kgs, this engine is widely used in the hovercraft world and is powerfull, reliable, easy to overhaul. And there are many other engines available.
I see that when you think gyros, you stay in the Bensen idea.
In fact, all the atual frames and rotor designs come from dr Bensen , right.
Thanks