Hello,
I've got an old Trek Antelope 850 which has a dead RST fork on it, and want to swap it to a fixed fork, so I can use it to commute. Where can I find a threaded fork, new or used, that will fit on my bike, as the head tube is quite long. Am I better off buying a newer frame with 1-1/8" headset, and more modern parts to replace the ones that do not fit? I also want to buy an adapter so I can use a 1-1/8" stem, as I want to use the stem/bar combo I already have. I would like to do it as cheap as possible.

Key point is the old fork's axle to crown distance. The only cheap forks (less than $50) I can find are ~400mm axle to crown, I don't think that is the proper suspension corrected length.

There are 26" rigid forks with 1" threaded steerer:
https://www.universalcycles.com/shopp...1&category=703

Here's a Tange with 1" threadless steerer, you'd need a new headset too:
https://www.universalcycles.com/shopp...8&category=703

Am I best to just find a new fork and frame? How would I measure my existing forks axle to crown distance? Thanks

You can run a 1.75" tire up front and a 1" in back (assuming you'll be running slicks) and that'll ameliorate non-corrected fork steering geometry problems somewhat.