There are a few things to consider.

What is the rpm range of the airplane engine (about half of the 21 buggy)?

What is the effective output (hp and torque) of the airplane engine?

Most airplane engines (non-comp engines) are designed to be extremely reliable, easy to fire up, and easy to tune. This results in the engine timing being more conservative than that of a competition engine. The result is an engine that only revs in the 26K range, will enough torque to turn a prop all day long.

I say, experiment with it (and your gearing) and have fun!