Now don't quote me on this because it is coming from memory, but I believe the HP rating for a combustion engine and electric motor are specified differently. I believe an electric motor is rated on the amount of continuous power it can deliver while a combustion engine is rated by the peak power it can deliver. That difference is very important.

So if you need a 200HP combustion engine to make your aircraft fly, an electric motor that will give you the same performance will have an HP rating lower than 200HP. Really you have to get into the mathematics of the whole system. You need to figure out the amount of thrust you need that will result in the airspeed you need to produce the amount of lift you need to get the payload(aircraft+people+stuff) in the air.

Once you have that thrust you can calculate how fast the propeller needs to spin(RPM) to produce that thrust, then check the datasheet of the electric motor to get an estimate of the current draw at the calculated RPM and a given voltage. Expect the actual current draw to be 10-20% more.

What are you thinking about doing f_lunn? I'm an electrical engineer and pilot, so I like where you seem to be heading.