Update: the fork, headset, and brake worked great. But in the end, the mismatch between the frame and fork led to rather unpleasant handling. So, I stripped the bike and will find another solution before next winter.

And an addendum: I was talking about this bike to a shop guy and he said that they called bikes with front disk only “mullet” bikes. You know, business in front, party in the back!

Edit: I decided to get a new Trek 520 frame. It will work perfectly for my needs, and it frankly is one of the very few modern bikes that has any interest for me.