Simplest. fork stays in place, headset remains. as before

Quill stem is replaced by an adaptor , it is quill on the bottom..

and a Place to clamp the threadless stem on top,

+ a threadless stem of your choosing..


Cane Creek was the developer of threadless headsets.. a type which others followed..

A US company Chris King makes sealed bearing headsets.
his company does have a design where the threaded 2 nut,
and the threadless preload are a parts change, on top of the top race.
but it is a much more expensive component..