My brother Daren had a friend in Birmingham who was very much into BSAs, also owned a BSA engined 3 wheeled car called a Berkley, I know nothing about other than one had to lift the bonnet up and climb inside to kick-start it, awesome!
Just googled this, and it wasn't a BSA engine at all, it was an Excelsior talisman 328cc 2 stroke, sorry.
Regards
Jase