I think there has to be an element of historic rights, people did just build "Specials" and there were no rules and it wasn't a big problem. In the classic car world here in the UK our 2 big problems are:
Blatantly modern vehicles being passed off as "historic" to avoid tax and MOT requirements. These vehicles are not modified in anyway but they are on false documents.
(I reported a 1959 Triumph on e-bay that was blatantly a 1980 Harley and the seller thought this was just fine)
Classic cars being converted to EVs with typically much higher (up to 10 times as much so a 65hp Series Land Rover gets 600hp) and being registered as only having one modification - ICE to EV power. These EV conversions bear no relation to the original vehicles in how they drive or how they are used.
What ever Bustard is, one thing is certain, it is an old British bike and that's fine with me. Sooner or later someone will turn up with a 200hp EV BSA and claim it is "better for the environment" and "the way forward"