There's usually enough thread on new threaded forks to cut off 20mm, but the best option is to email the seller and ask the height of the lowest thread, or the length of the thread measured from the top, and compare it to the height of your threaded headset cup.
If the fork is still in the bike, measure from the outside to the lip of the cup. It's isn't perfect, but hopefully you won't be right on the line, so the rough measure will tell you what you need. If it's borderline, you'll have to remove the fork to be sure.
If you opt to go threadless, you'll need both a headset and stem. Any standard 1" threadless headset will work, you can't use any integrated or "hidden" headsets.
Good luck whichever way you go.
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