If it came with a knob, very likely the knob doesn't mount the same as the original. You might be able to make up an adapter, though. I have one of my TR3 knobs mounted to a piece of hex computer board standoff, with a tiny setscrew in place of the spring pin. If the aftermarket knob doesn't unscrew, you may have to break it off and thread the end of the shaft for the adapter.
Remove the inner cable, and if it is stranded, wrap a turn of masking tape tightly around it. Then cut it slowly with a Dremel and abrasive cutoff blade. The cable is hardened spring steel, so ordinary cutters cannot cut it cleanly (and will be damaged by the attempt). The same tool will also cut the outer housing cleanly. Dress any sharp edges before you put it back together.