Thumbs,
My rule of thumb is
the gas engine should have 150% of the displacement of the recomended glow engine. This is because the average gas engine has 2/3rds the power of a typical glow engine. It is just like the glow 2 stroke to 4 stroke ratio. You put a .90 4Cycle in a .60 2 cycle plane. 3D is a different story and Surfer is correct about the weight issue. Wing loading has to be in the correct range for the size of the plane.
Dave