I'll back what others have said, either arrange for the original to be 'forge welded' - but the piece may need to be heat treated to give the required mechanical properties - or 'drill out' the old shaft and replace it with a new piece - either hammer-forged, as the original or machined from newer materials, to give the same effect.
DO NOT EVEN CONSIDER recasting the shaft, as the casting will be brittle (even after heat threatment it will be relatively brittle compared to the original) and likely to shear in service (fairly early on at a guess).
Sorry I can't help more, my fields were corrosion and (later) wear.

Regards

Walrus