As long as you have the threaded top nut, you can replace everything else with a threadless headset, like an aheadset or something. That leaves plenty of room for the clamp thing. I did this on two bikes, and still have all the bits I took off. I'll try to remember to look for them.
I'd looked into a modern threadless headset as a solution, but if I were to replace those old bearing races with a threadless headset, what's to keep that top nut from unscrewing without the other nut? That's the problem I'm having now, and from what I can tell, the setup that's on the bike right now should be about the same as an Aheadset, only much crappier.
I suppose, I could put a threaded top race on there, from a different Raleigh (if they're the same size tube), with a washer, and that would solve my problem of the fork coming loose in the meantime.
EDIT: No, this won't work, since the threads on the tube don't go down far enough. But I might still be able to use a threaded race in the meantime in place of the missing nut.
I just took apart my front end, and it turns out that the 'missing' chrome cap (the one that goes over the nylon bushing) is actually what is sitting on top of the race in my photo! I don't think it can be reused for it's original purpose though, because it's pretty badly distorted from being misued.
I'm willing to explore the route of a modern headset, but I'd prefer keep it stock, since I know that setup works fine for me. I also think the bike is missing a bit of it's Raleighness without the Raleigh heron bracket on there.