I came across the same problem a year or so ago when I was repairing a ham radio transceiver, there is a solution but you will need to hunt for some parts:
1) Find a rotary switch that has the correct number of poles and connections, the switch should be of the 'open frame' type.
2) Disassemble the switch and remove the shaft. In my case the shaft was 6mm but yours will probably be a quarter inch.
3) Replace the shaft with a short length of aluminum or brass tube then reassemble the switch. A certain amount of mechanical ingenuity is required here and every situation is different, in my case the main problem was filing away the shaft to fit the C clip without weakening it too much.
4) Obtain a potentiometer of the right value but with a 4mm shaft. The shaft will probably fit down the tube used on the switch but if it doesn't then it is easy to drill out.
5) Assemble using custom bracket that holds the rotary switch and the potentiometer behind it.
All of this work took me three days but it was worth it because the switch/potentiometer was unavailable and I needed to get back on air.