Carb would be a 10TT9 Amal but a 389 monoblock or 930 concentric would make life easier.
Check out
www.hitchcockmotorcycles.com or
www.burtonbikebits.net.
The Hitchcock site has a bulletin board where the experts "over home" will have all your answers since they were, and and still are, pretty common in Britain.
Don
1965 Interceptor
1970 Interceptor