Good evening, Sir;
I realize this isn't going to help
much, but here it is. Perhaps you can take this a little further and get a good fit-up.
The gears are(should be) Metric modulus one(1), 20 degree pressure angle, and an eight(8) MM face with a 12 MM bore. There is one pair, 127 and 120 teeth, that are basically an idler and metric/SAE conversion set. Although it is always in one or the other configuration, it
can be changed, giving you other (unlisted) threads.
My machine has a QC gear box, so I can't
reliably speculate on the tooth count you need. Both Grizzly and Harbor Freight are possible sources. Likely cheaper, but may need touching up a little. The given size is fairly common. You might try other sources, especially as sets, once you know what you need. They are used on many(if not most) of the lathes that size. The Grizzly model is G-4000(currently). The HF model I don't remember. But they're the same machine. There are many others, I just happen to remember those two.
There is one plastic gear that operates as a "safety valve". That one you need. The rest are steel. I wouldn't recommend printed synthetic gears, although they will work. They just won't last as long.
Your threading/cutting cherts should reveal the tooth counts you need. Keep in mind too, any "mis-fits" will change the threading. Often not that you can see, but it will be incorrect. If the chart calls for a 43 tooth, a 42 or 44 will fit well enough, but won't be
quite right.
I realize this won't help a lot, but should get you started on the right path. I do have a related project that you may be interested in.
http://www.hudsontelcom.com/9X20Gear.html It will give a little insight into your machine.
Bill Hudson