you could use a regular pot, and solder wires from the pot to the circuit board. I fixed a red knob champ 12 using this method.
Are you absolutely sure that the value is 100K and not a 50K I have a fender frontman 212r amplifier I am replacing all the potentiometers, the volume potentiometer I have removed is a B 50K. To access the Board to remove the potentiometer you should snap shot your wiring the, power wires take some finesse at the plastic clamp and the same applies to the speaker wires. You should invest in a [solder sucker, makes it easy to remove the solder from the Board and free the potentiometer from the board.