If the information was available then choosing an engine by torque is preferable to choosing by HP. Almost all engines develop max HP at unrealistic revs for normal sport flying.
But I must say that w8ye would lose his bet by a big margin Put a small enough prop on any engine and it'll breeze in 16K. I'd guess maybe an 8x4 on an LA40 would do the trick let alone on something like a Rossi. As for flat out with a flywheel...well that really depends on what's the weakest point in the engine. If it doesn't break then the revs are only limited by the (reducing) torque available to overcome friction. Ever had, or heard, a shaft run? Wowie!