Welcome to the forum! This issue has been discussed many times. Once the knob comes off, that's pretty much it. There are no aftermarket knobs available. A few options are:

1) use a volume knob for a guitar. I've got one on a Zoomatic that has a beautiful abalone insert on the face. The knob is held on with an Allen screw.

2) This is the method I've used quite a few times. I obtain a large Slingerland tone control knob and drill it out to 5mm. (tough to find a 5mm drill bit locally, so I had to get one on eBay). The 5mm size will work as long as the splines on the end of the shaft are still good. (the splines on your Zoomatic shaft look good). I use a small washer in between the knob and Zoomatic body and then press the knob on with a vice. Just enough to to leave some clearance to let the knob/shaft work properly. My pictures should tell the story. Good luck!

-Mark

Mark, I like the muffler knob fix. Good thinkin on this one! Thanks for the details too. Many poor zoomies out there need this.

Glad to help! And, yes, way too many "poor zoomies" out there!

-Mark