The only way to change the choke or vent knobs is bust them up (I used hack saw a couple of times) chip away the remaining plastic. I then buffed the cable end with a wire wheel to make sure all residual plastic particles were removed. I then used epoxy glue to fasten the new knobs. Make sure Rudy still has the knobs before breaking the originals! I did mine back in 2005 and don't know if he still sells the knobs.
Original knob -
Cable after knob was crushed off/removed -
And with new knob installed -
The Vent and Choke cable ends do have a slight grove in them where the knob fastens and this goes to the top so you do have to make sure the knob is centered with that so it looks correct when re-installed.
New knobs really look nice when they're installed
In '05 the new knobs were $18 apiece and have likely gone up since but I had no problem paying as I remember when you couldn't get them at all.