There are zillions of 126mm freewheel hub wheels around. I'd go with that rather than respacing. I have a 1985 Trek 620 which gets ridden a lot, fwiw.
On mine, which one day I may paint and restore, I just sand the rust off and brush on some paint that sorta matches on it, again, telling myself that one day I will give it a nice paint job. If that is too much, sometimes I've been known just to put some grease on a scratch down to bare metal, that excludes air/water for a pretty long time until I can get out the paint/brush.
I love my 620, battered veteran that it is.