Joined: Nov 2006
Posts: 19,810
Likes: 597
From: NJ, NYC, LI
Bikes: 1940s Fothergill, 1959 Allegro Special, 1963? Claud Butler Olympic Sprint, Lambert 'Clubman', 1974 Fuji "the Ace", 1976 Holdsworth 650b conversion rando bike, 1983 Trek 720 tourer, 1984 Counterpoint Opus II, 1993 Basso Gap, 2010 Downtube 8h, and...
A few options:
--NOS Raleigh headset off ebay (good luck!).
--Used Raleigh headset off another Raleigh (post your need in the C&V marketplace)
--New 24 TPI fork and new headset to match.
Beyond that, things get more complicated. What's wrong with your existing headset -- specifically, how's the threaded fork race? If you can find a new headset that will work with your existing threaded race, you can keep that and everything above it, just replace everything below that. The new headset can be any threading, including threadless, since you won't be using the threaded bits.
If the threaded fork race is shot, keep your threaded top nut, and replace the four races with a threadless system (Aheadset). But you'll need a second threaded nut to lock against your existing top nut. You won't have room for much, but it doesn't have to be very tall; maybe 1/8" would do it. If it's very thin, you can get by with a 24TPI piece. Possibly you can grind your existing threaded race down to one 1/8" slice with the right threading. This will take a while.