Keith,
Interesting. What number spindle does your lathe have? A number 5 size has a spindle bore of approximately 1.25" stock and fits 5C collets. A number 4 is approx 1" stock and fits 4C collets, 3C I am not sure but fits 3C collets. There are 4 deg taper and threading spindle nose variants for the outside of the spindle noses.
There is a chap that is a regular poster on the practical machinist Hardinge forum who is a fountain of knowledge on Hardinge manual lathes and mills, especially the older models. His name is on the lathes.co.uk link Michael posted. I expect if you posted there you might also have chance to locate a replacement cross slide and possibly other accessories.
Steve