Id say ream out the top collar so It will slide over the new threadless steerer..
or just get something else that will ..


sort out the collar at the top of the top race. it will sit on top, then the spacers
sit on that and the stem atop the spacers ..

though the Sheldon kludge is interesting, you have to find someone with a cutting die for 9/8'' steerers.
that would be perhaps even harder than finding the fork.

if you are going to go (sorta) threadless ... just go for the full conversion it's more practical .

a bike shop can pull parts out of stock bins and see what works