You may already have a foot pedal. Sometimes the pedal is sitting inside the knee lever bracket. Give it a look and see if it is in there. If it it pull it to get it out of the bracket. It might be stiff, it has been in there for 50 years!
It will be easier to get a new tension unit than to find the parts for yours. Go on Ebay and look for a tension assembly for a class 15 sewing machine or even the complete faceplate.

Tracy, I am uploading a picture of the knee press, not sure if this is the foot pedal that has been attached to the side.