Kansas Motorcycle Museum
This is an item that one could purchase for their bicycle through Sears & Roebuck by mail order in the 1940's when you couldn't afford a full-on motorcycle.
If you look closely you can see a foot operated lever on this side of the bicycle. It is connected to the coaster brake in the rear hub so you could stop the darn thing since you had to remove the pedal and crank assembly to mount the motor. Normally you would reverse your pedal action to slow down and stop the bicycle. On the opposite side there is another foot pedal which operated a clutch assembly (that round drum thing) so the bicycle wasn't in constant motion. The sprocket on the rear wheel looks like what would have been the sprocket up front where the pedal cranks were. Obviously the motor turns over at a higher RPM than one can pedal a bicycle so the gearing was reversed and you could ride at a reasonable speed. A cable runs up and into the handlebar to a twist grip like a motorcycle which controls the throttle.
Pretty clever all.
My only concern is your trouser leg getting caught in that whirring rope pulley on the motor.
It'd rip your britches right off of you!