Take an ordinary paper clip and wrap it wire. Put the bottom of the magnet, apply a current to the clip and simplest motor is ready! The same principle lies at the basis of wheel-motor for bicycles, but instead clip the wheel. The same can be used in a small airplane.
Although it is well known that the same size engines can differ significantly in power. This is due to the fact that they use different magnets and different wire thickness. An ordinary magnet can lift 1 kg of weight, while there are rare earth magnets, which are able to lift 180 kg at the size of a coin.
Accordingly, the power of the motor depends on what kind of magnet it is, because if you apply to the wire a current produced a magnetic field. Therefore, based on the size of the intended or existing aircraft, select the appropriate magnet for a motor.