I think you should make sure exactly which model Austin your Mum has. Austin Sevens were not made as taxis, far too small and only 2 doors. A prototype A7 taxi was made in the '20s but never went into production being unsuitable for use as a taxi.It is possible that a Seven was converted for use as a taxi by a garage/bodybuilder but wouldn't be a UK manufacture. Export models of Austin Sevens had an air filter fitted and if it is a Seven you would probably have more chance of getting one through an Australian/NZ A7 Club. They weren't fitted to UK models so are like hen's teeth here.