Remove the small set screw from the knob and if it still won't unscrew, take the needle out of the bar, and the presser foot off, put a small block of wood under the needle bar, and as you unscrew the clutch knob it will cause the needle bar to hit the wood, then put more force into unscrewing the clutch knob. If you still can't unscrew it, protect the knob with a leather strip or gloves and use a big pliers. It also helps to put tri flow on the area stuck.
When putting the clutch knob back on, I never can seem to get it right the first time. If it is not working properly, take it back off and turn the lock ring so the notches are re-aligned and try it again.
Good luck with it, may you treadle on!
Ethel