Gasoline has a much higher fuel-to-air ratio than glow fuel, and thus if you compare a gas engine to a glow engine of equal displacement, the glow engine will provide more power. Based on the aerobats I've looked at over the past few months, something that a 1.80 four stroke glow would power would be happy with a 40-50cc gas engine.

Of course it depends on the type, size, and weight of the plane.