Sew Classic has a repro hand crank for $15 plus shipping:
http://www.sew-classic.com/
You can also buy a double lead cord, foot controller and even a light which is all you'd need for an "easy" electrical set up.
Knee Bar will run you at least $40 and you have to make sure you get the right one. You'd also need the connectors and power cord to work with the two prongs you pointed out. Usually the controller is so old it needs fooling with. I'd only go this route if set on restoring to original working condition.
Good Luck
Jon