Head bearing lock nut thread on a vintage english bicycle
You really should measure it to be sure, using a thread pitch gauge and caliper, but if it's made by Raleigh in that era it's probably 1"x26tpi. The frame bore and original stem OD are probably standard dimensions (30.2mm and 22.2mm respectively), and if they are then you can just use a new standard ISO 1" headset along with a new fork that takes standard 1"x24tpi threads.

I've avoided doing this judiciously enough that I don't really know the practical details, but it's common to hear of attempts at extending threads by very much using common dies and a handle going pretty rough.

Basic 26x1-3/8" replacement forks are still available new. In the US, any shop can get you this one. It will have the standard 24tpi threads, so if your headset is 26tpi it will need to be replaced.