I added a suspension fork to my Trek 800, which had a 1" threaded steerer. I bought a 1" threaded fork, but the threads didn't extend far enough. Couldn't return it. Unable to cut more threads because the metal was too hard. I converted it to 1" threadless. Installed the appropriate headset and new crown race. Replaced the quill with new stem, spacers, new handlebar, new grips. All of this cost far more than the bike was worth, but I like my old Trek.
For serious mountain bikers, there are technical issues about rake and steering geometry to think about, but I just wanted to start my bike rides rolling off my curb smoothly. The steering feel was changed, but I got used to it after a day, and it does soak up bumps, for an inexpensive fork.
I did consider a conversion headset. Some exist. This guy had a threadless headset that he turns down on a lathe that might fit your bike.
Maybe he can modify a threaded unit for you. There's an install write-up in the above link. Here is a another writeup
from a customer. You do have to think about the strength of the bearing cups after the metal is removed. Maybe he starts with parts that are purposely smaller.