Can you afford a new threadless headset and a couple reducer shims.
Either the shim, there are machined ones for the purpose,
goes inside the current headset
Filling in the 16th of distance, to slip in a 1" threadless fork,
or the shim goes in the frame and the new headset fits against the shim.
then maybe you can kludge in a threaded headset , if you got a threaded fork.