Richard,
If you find an old pump and go down the rebuild route I can highly recommend Holman Engineering in Addlestone Surrey.
precision@holman-engineering.co.uk or 01932 353555. They are an old school father and son concern who specialise in rebuilding water pumps, steering boxes and other parts for vintage vehicles. I had the pumps for my Carrier rebuilt several times by other less reputable firms who used modern seals none of which lasted, I then bought a new rebuild kit of the original type using the carbon disc seal, the Holmans machined the pump body and fitted the new parts correctly and I have never had a leak since. Several of my friends have now had their pumps done by them with similar satisfaction.