I guess I'm spoiled when it comes to machineists. We have 2 local guys that are like the professor on gilligans island.. they could make a nuclear reactor out of some coconuts and string.
The one guy runs a pnuematic and hyd's repair shop, and it is nothing for him to make new input plates or rings for pumps, or custom cylinders with spool valves machined out of a solid block of aluminum, in one day. He's done it for us on more than one occasion.
The other guy has a mobile rig.. looks like an old ups or bread truck. Has a lathe/mill, generator and welder.
He's come out to our jobsites in the morning, and line-bored and sleaved our machines on the job. Once he had to make us a set of gears, and a slack adjuster for a chain driven torque converter. He just drove out there.. took some measurements, grabbed a hunk of steele from his scrap pile and started to work, and was finished before the sun set.
Of course.. both these guys services come at big cost too...
As for helical gears..... do you think gear noise is going to be heard over the tractor an pto driven device?
I've driven fords that had the stright cut gears and realize that you always think the rear end is fixing to drop out.. but heck... the price difference may be worth it.
Some of the parts you mention may be easy to locate / fabricate. Splined couplers could be the basis for a mount for the drive gear. The output could incorporate an ORC adapter that uses a smooth input shaft / w key.. or just weld up the output shaft to the ORC input. Lotsa ways to go about this.. still a bit of money either way.... just depends on if a new tractor is cheaper than making the old one work.
Soundguy
