You can get the replacement rubber tire and knob on ebay. People can and do part out these machines that are too far gone for saving(or because they can get more to part them out than to sell them:<) But, if you're patient you can probably find your knob. The knob tightens, so your stitch is "locked" in. Not absolutely necessary, but a good thing to have so you don't accidently move it while sewing or the vibration of the machine may move it and change the length on you.
Great find!