Yes, that can and has been done, and works fine, but the power is still not that great. Definitely not enough for a good aerobatic plane. The little 18cc weedeater motors have very small ports, low compression, and are cheaply built. They are designed to run in a $50.00 weedeater that is mass produced and you can hold the throttle wide open all day and it won't hurt itself. Better like you said, put it in an airboat or a little car than an airplane that needs a lot of power all the time. In the hot rodding world, we always say "The only substitute for cubic inches, is more cubic inches." If you want a good aerobatic plane, you are going to have to go to a larger gasser conversion engine.

Good luck,
AV8TOR