Step 1: Can it be fixed?
Step 2: What you'll need.
2. Brass or Aluminum Tubing
3. All Original Parts to Broken Knob
The pipe cutter was about 5 bucks. Get the tubing at a local hobby store. Take the knob with you. The diameters come in increments of 1/64th of an inch (I think). Select the size that fits snug over the cylinder of the knob.