I will address your question as if this tractor were mine.
1) First, I would try to find a good used one, which maybe hard to do. If I could not the next thing I would do is this…
2) I would start getting pieces to make a new shaft assembly. I believe a shaft assembly 12-13” long is available on the market (Surplus center) I would turn down this shaft to duplicate the original.
3) The center hub that mounts on the crank, I would bore/mill out the center and buy a 6 spline weld center and repair the splined part. This piece will need to be carefully welded in place to assure strength. I may also opt to find that piece used since it is critical as the drive unit.
There is also the option provided you could find the 6 spline broach of making the hub piece completely new from 4140 pre-hard or similar steel.
P.S. you can not whave the spline area welded to the hub the crank needs a little play that the spline provides. Its better to copy what you have.
just my thoughts