Not 100% sure, but the various straight blade forks are very similar. They are all non-suspension corrected as far as I know, so probably below 400mm axle to crown, likely 390mm-ish. I have an Alien, had an Apex and have the fork off a Colorado I think.

You would have to add threads to go down a size I think. The Colorado fork I have was off a very large frame and I cut the threads off to use it as threadless, if I recall the threaded section was on the short side. It was/is very rusty so I don't feel super bad about that one.

Would gladly sell you the Colorado fork, but it's very sad looking and now threadless. The Colorado fork seems very light to me, and certainly rides like a nice fork. I would have thought it was cheaper/heavier coming from a lower model. Have not taken the Alien apart to see if the fork is lighter or different tubing.

Maybe check retrobike.co.uk for information, the archive sometimes has catalog scans when the forums fail. I'd go measure some forks, but everything is in storage.