That depends on a few things:

- the engine (sounds like a Ryobi 31cc conversion)
- the airplane
- your definition of "being able to handle", and "underpowered"

I used a Ryobi conversion on a Dynaflite Super Cub and a Chipmunk. They both flew just fine.

I would not recommend such an engine on something like a CAP, Extra, Ultimate, mainly because of low power to weight ratio.