I will second the thoughts of using a 4-stroke. Enya, Thunder Tiger or Saito are all very good choices. Otherwise I would go with a nice 60 size two stroke like the Enya or Thunder Tiger.
I have a Great Planes Sportster Bipe and it is under powered with a 46FX on it, even though it is light, it just has a lot of drag. My Saito 91 will be going in it for next season.
Remember you always have the throttle.

Fuelman