Unobtainium headset size? This summer I took apart my old mountain bike to do a rebuild. It is a Raleigh Legend, from Raleigh Canada, circa 1994. When I got to the headset, I looked for a replacement but it seems the combination doesn't exist. It is a 1" threaded headset, but the diameter of the pressed-in part of the cups (and corresponding ID of ends of headtube) is 34mm. It seems headsets for 1" are intended for 30mm cups, and to get 34mm cups the headset is based on 1-1/8". I thought perhaps there would be a way to deal with this with circular shims, and I do recall seeing some online, but as I remember there was still an issue with the crown race ID, so in the end I reused the headset parts but replaced the balls.

So the question is, is the only way to put a new headset on this bike to replace the fork with one that has 1-1/8" steerer tube (which also means new stem and possibly handlebar)?
The product you are looking for is called "headtube adaptor" or "headset reducer". Once you know the name, you should be able to find them online. 

If the headset is in good shape, there should be no reason to replace it. Rebuild usually refers to cleaning, greasing and adjustment, and possibly replacing bearing balls.