This would be a good powerplant/drivetrain combo, a Honda 50cc motor with an internally geared hub, chain drive.
Motorized Bicycles Detail Page
The gearing offers you the ability to run the motor at the right rpms for most efficient fuel consumption. I believe it also has a freewheel on the hub so that you can shut down the motor during the glide and the chain and clutch do not turn and cause drag. By changing the size of the output gear, you can easily adjust the range of gearing to fit the drag of a faired trike, in other words, gear it for lower rpms at a given speed. The kit is set up for relatively easy installation on a bicycle frame. It also allows for pedal assist which is very helpful starting from a stop, where the motor is usually the least efficient.