The Sick shifter kit together with a 50cc 4 stroke HuaSheng (or similar) motor would be a good way to motorize a tadpole recumbent bicycle. Add a coroplast fairing and you are in business.
Sick Bike Parts
The shifter kit allows you to transmit the motor power through your derailleur system. It allows for freewheel coasting with the engine on or off. It allows for pedal assist at lower speeds. You are not required to pedal at all, the freewheel system lets your pedals "coast" while the motor powers the chain and wheels.