you can always fall back on 746 watts of output equal 1 horsepower output, and using that, calculate your effective horsepower via the rpm and loading of the prop.

that said, there is no easy formula to give you drop in equivilents for glow motors, and it is further complicated by the fact that e planes can be built quite a bit differently then glow. (no extreme vibrations trying to destroy your plane, and your weight generally is further back)