Further to reply in other thread the rim size for a direct replacement is ETRTO 451 such as:
https://www.sjscycles.co.uk/rims-20-451/
You might be pushed to find complete wheels in this size with a SA hub?
Dawes Kingpins also used this size/hub so could yield donor wheels but also a very nice bike.
Tyre fitment is more limited than in 406.
....argh, took too long and
@Guinessisgoodforyou was too fast on the draw!!
I've started so I'll finish.
You 'could' try a donor 406 wheelset and see if the brakes will reach?? The mudguards won't fit as nicely and you may need to adjust the stand but it could open up more options if 451's are a no go for whatever reason.