Small motorcycle engines would be ok. for power, but they tend to be rather heavy, and of course have to be seperated from the gearbox etc.
The main reason for their weight is that they usually have very heavy, full circle double sided cranks, (very strong too). Light weight is not so important for motorcycles as model aircraft.

The chainsaws and weedeaters have to be lightweight, and usually have bobweight cranks, and many weedeaters have single sided overhung/cantilever cranks, which reduces the weight even more.

The chainsaw and weedeater types also have pumper type carbs, which allow them to work in any position, as opposed to most motorcycles which normally have float type carbs, and will not operate the inverted position.

All of these conversions suffer from having large flywheel type magnetos for spark ignition and this also doubles as a cooling fan, so the cooling fan vanes need to be removed ( carefully with balance in mind).
This heavy magneto flywheel can be removed if you fit it with CD type ignition and a battery pack.

Buy a cheap used one, and give it a go! - plenty of advice on the forum.