1” MTB forks do seem to be hard to come by, but not THAT hard. I helped a friend update his Mongoose MTB that had a useless “shock” fork. I bought a used Tange unicrown threaded MTB fork that had the correct Canti bosses for like $30 + shipping. He was excited that the end result was lighter in weight & functioned adequately for the light dirt trail and bike path riding that he did.
If it were me, I would keep looking for a proper replacement fork & leave that (complete) Trek backup frame set alone.
i would only scavenge that fork as a last resort.
When I haven’t been in the market for a particular part is when they suddenly become very available. Some sort of cosmic irony I guess.