Its expensive,but StatonInc sells 20" & 26" rear wheels with freewheel threads on both sides. Before someone says that its just a 'flip flop hub',it has left hand threads on the motor drive side. That way,when yoy pedal,the motor side freewheels. This means no drag.
It says pre made wheels,but that's a lie. You have to add the proper hub to the rim.