I would try the engines used in the RC-cars, some reach up to 40,000 rpm just like that. Some fellows may say that those are small displacements, but that is on my opinion good if we consider the Diameter of the Propeller and the Linear speed of its tips. Even with 6 inch propeller 30,000 rpm gets the tip beyond the Ultra-Sound speed and to ... dangrous desruction. So I would ask the Question: - Yes, Fast Engine, but with what kind of Propeller ? - Well, mabe Impeller then but that is 'a bit' strange use of combustion engine, isn't it...

Cheers, Nick
(also OS fan, beside the Italian SuperTigre an Rossi of 'good old times')