Is your current fork in good shape? Just find a person that does brazing or silver soldering and have them cut the steerer tube off a few inches above the fork crown and then use the unthreaded part of a steerer tube from some other junk fork to have brazed on to the stub of the fork you wish to use. You just need to fashion a union out of another small section of steerer tube by splitting the tube and making it smaller to fit inside the steerer tube where they will be joined.
You might get less naysayers about this if you go to the
Framebuilders forum and ask. For certain you need to have this done by someone that is competent in brazing and this type metal work.
As for the headset, you'll have to look, they are out there, but not in great quantity or selection. However the treaded headset on my '78 Raleigh Competition GS seemed like it'd work for a threadless with a little modification. I never found out though as I bought a new bike and that project when by the wayside.