there are two knee levers. Based on age, I'll guess yours has the FEMALE end - that slips over a pin in the case. The later style has a male end, and is much more common to find. There is a third style that has a female end but a straight bar that goes with the cabinet style - but that one is really rare.
The second style came out around 1929 and into the mid 30's. With this style there is usually a bakelight plug on the back of the machine for the cord. With the older the motor is hard wired into the case.
Expect to pay $30 - $50 for the lever, if you want an authentic one.
If you do not want to pay - you can also convert the machine to take a foot pedal. If you go that route, I'd suggest mounting the bakelight plug and wiring the machine to that, and then use the two wire cord that has the foot pedal attached.