That's a nice little history from where the tractor came and where it is now. I'm not familiar with Newfield or the scrap metal yard as it is nearly couple hours South of me.
It will be nearly impossible to start the hydrostat tractor with the mowing deck engaged, especially in the freezing temperatures. If you can't disengage the PTO by pulling back the PTO handle located on the right side of tractor near the steering wheel, for now I suggest you remove the belt that runs from PTO to mower deck. Heck, if it was me, I'd even remove the mower deck because it winter season. The PTO could be frozen up from rust or maybe you have worn or missing parts (PTO button, link rod, etc....)
Manual for you tractor:
https://www.onlycubcadets.net/forum/...ead.php?t=4642
Wiring diagram link:
https://www.onlycubcadets.net/forum/showthread.php?t=35