Building electric trike

Trikerider

Almost there, ready to add crank motor. I need help with hub see attached pic.(Big wheel for adults). The wheel is 20 inches, uses a 103mm axel, standard 3 speed freewheel hub and the crank is attached to a single speed cassestte. What I want to do is add a chain to the motor, but Iam not sure what setup I need so the crank/peddals do not move when the motor is running.Any help ideas so I can complete this project.