You can simply adjust the maximum wattage being produced by the motor by limiting the current:
As
Watts = Volts x Amps, then
Amps = Watts/Volts.
This makes it relatively easy to calculate the required current, but you need to add ~25% extra to allow for the motor's efficiency of ~80%.
(The power consumed by an electric motor is always greater than the actual power it produces.)200Watts/24Volts x 1.25 = 10.42Amps
(10Amps @ 80% efficiency = 192Watts, 11Amps @ 80% efficiency = 211.2Watts)200Watts/36Volts x 1.25 = 6.94Amps
(7Amps @ 80% efficiency = 201.6Watts)200Watts/48Volts x 1.25 = 5.2Amps
(5Amps @ 80% efficiency = 192Watts, 6Amps @ 80% efficiency = 230.4Watts)I can't possibly see how the authorities could accurately measure the power output from your bike, so you could simply set the maximum current slightly higher and explain that the motor is not very efficient:
If you run 10Amps max @ 36V it will actually consume 360Watts of power, but if you had a motor that was only 55% efficient, then the actual output would only be 198Watts!
I hope this helps.
Alan