As with most postwar Villiers engines there are two wiring diagrams available. One the “rectifier” diagram that provides for charging a battery and thus having a steady lighting plus a stop/brake light. The second the el cheapo “direct” no battery lighting system where the degree of lighting available rises and falls with the engine revs and fitting a stop light requires a bit more shenanigans.
Both are archaic and my electronic engineer nephew positively gurgled with horror when first I showed the two options to him. Most people find it no great drama to update and vastly improve Villiers electrics using modern electronic components.
Francis Barnett used these same Villiers diagrams. There is no special Francis Barnett wiring drawing. Both Villiers Mk 30C Operators Instructions and the Francis Barnett Plover Owners instructions are cheaply available from multiple sources, I’d suggest Villers Services for the engine as you’ll be bound to use them sooner or later for bits and bobs and the UK National Motorcycle Museum for the Fanny Bee handbook. The NMM are very handy for overseas buyers as they will send an electronic download. You’re not waiting weeks for a book coming through the post.
Best of luck.