Threaded, I assume? It's a case of searching the Bay Of Thieves for one with the right steerer and thread length. If it has a funny offset (race forks are almost always 43-45mm) you'll probably have to get one made. Then there's paint, unless you get a chromed fork.
Argos will make a nice new one for £250 including paint, which could be competitive with buying a 531 fork from eBay and having it sprayed. A new steerer is about £60 (plus paint) if you can't find one in the right length.
I can't believe the wheel crumpled like that. Radially, a wheel should be stronger than the frame - or roof rack!