I would suggest that you might try contacting Bishopgray and see if they can help you.

There is another potential issue with the fuel switch in that the internal hole diameters are slightly too small and can restrict fuel flow particularly when running for long periods with large throttle openings such as on a Motorway... which probably wasnt a big problem in the 1920's ! While you are working on the switch, unless this has already been done, it might also be worthwhile trying to see if you can get the holes enlarged.