Hi tbub!

I doubt you'll find a knob. Even if you do, chances are that it won't stay attached. They are a press fit. Once they come off, it's pretty tough to get them to stay on. Jim Petty, of jp2creations.com, offers a complete rebuild, with a new knob, but it is pricey at $100.

I've had good luck with installing those larger Slingerland muffler knobs onto the Zoomatic. See pictures. I haven't done this in years, but I do recall having to slightly drill out the hole in the knob and then perform a press fit onto the shaft. I don't recall what size drill bit I used. Once done, they never came off!