I just picked up a mid-late '90s Trek 850. From what I can tell the bike is a '96 or '97 model.
The bike needs some TLC. It has a threaded headset and worn out fork right now. The headset seems to be a little stiff. I'd like to move it to a threadless setup so that a 80-100mm fork could be fit. I'm not sure how to measure it to see if I can retrofit a 1-1/8" threadless headset. How do I go about this?

Would you keep it as a 7-speed or move it over to single speed? I already have an ancient Trek 930 SS that I love. The trouble with that bike is its ancient Rock Shox Judy XC and 1" threaded headset. It's getting to be harder and harder to find replacement parts for old Rock Shox. If I can install a threadless headset on the new bike, then I'd be able to enjoy a wider selection of forks. Plus replacement parts would be easier to find as things wear out.

Thanks for your help!