Do you know what models share this same PTO setup? I might just buy a used one if I can find one.
I'm not sure this will help you at all, but I'll try.
For the Bolens 600, the PTO control shaft is part # 1713369.
For the Bolens 650 & 750, the PTO control shaft is part # 1717347.
For the Bolens 800 & 900, the PTO control shaft is part # 1714589.
For the Bolens 853, the PTO control shaft is part # 1717324.
So, it appears the PTO shaft for the 600 is unique unless part # 1713369 was superceded by # 1717347. (Maybe someone with the Bolens Parts Price List can tell you.)
All of the tractor models listed above use a single belt for the PTO drive, and all use the same PTO idler pulley - part # 1713803.
If you try to fabricate a new shaft, the arm that the idler pulley bolts to will have to be the correct length, and welded in the correct position relative to the roll pin holes, so that the PTO lever functions correctly.