During my garage band era… that was a very common occurrence on guitars/bass amps knobs. What we used to do if we couldn’t find a replacement potentiometer is to use a small saw blade and saw thru the center of what remains of the knob. It has to be wide enough to fit a dime or a quarter to rotate the control.
Hope it helps...
~Jaime


The sailor does not pray for wind, he learns to sail. --Lindborg