Two things,
First, I'm green with envy that you have a 20DC. Being a Villiers nut I like all Villiers powered bikes but the 20DC is by far the prettiest.
Others may lust after Triumph, BSA, Notron etc but they are children and know nothing.
Second. The switch you are after is the bog standard Villiers switch fitted to ALL makes of bike that both used a Villiers engine and used the rectified circuit on their better models. They are definitely not rare. You are just having a bit of bad luck finding one.
I suggest you start contacting any of the Clubs that have an interest in Villiers engines, particularly those that are road bike oriented. The British Two Stroke Club, the Francis Barnett Owners Club, DOT Owners, James etc, etc, or seek out Villiers Services and buy one. These switches are out there. Good Luck.