If you want to actually estimate the power a glow engine makes, use a tach to measure rpm at full throttle. Now take whatever prop was on that glow engine and put it on an electric plane and measure how many watts of power it takes to spin it at the same rpm with a wattmeter. If the electric motor won't spin it that fast, measure the watts at whatever speed the motor can turn it and then take the ratio of glow engine speed/motor speed and cube it.

For example, let's say your glow engine spins a 10x6 at 12000 rpm, take that exact same 10x6 and put it on an electric plane and throttle it up, let's imagine that your electric spins it 6000 rpm and draws 150 watts of power while doing that. Take the ratio of glow/electric rpm, in this case 12000/6000 = 2. Two cubed is 2x2x2=8 150 watts x 8=1200 watts.

In this case, your glow engine could be called a 1200 watt equivilent motor. To know the actual output watts, you need to know the efficiency of the motor you measured the prop with, most likely about 80% so 1200 watts x .8 =960 shaft watts. Convert to horsepower by dividing by 746.