Hello Scott
What I would personally do (since you are using it and not a show piece) is put a later 70's white victa housing with paper filter for better engine life, plus easier on the wallet as well.
If worse comes to worse, throw on an easy to get Rover filter housing for now and try to track down a Victa one
Somewhere on here is the Victa bulletin saying 'we no longer make the old housings, use the white one in servicing'