I think you circled the six overload relays with heaters. Three for low speed and three for high speed. Why replace them?
You're treating the symptom and not the problem. Look at lower voltage problem first.
A heater is part of an overload device, they can burn out, but getting partial power sounds "funny". Your pics are coming out too small, and I need up closer pic to tell what I'm looking at, and any data that is on it.
As pointed out, that is an overload relay. Remove your jumper and under the screws should be heating element that goes down into the relay cavity. If there is none or the heater is burnt out, then that is your problem. Your motor is running single phase. Take one of the heaters out of one of the adjacent relays. You will need one like it.
Yes, heaters are still available, yes they are antiquated, just like the lathe. Remove old one, take it to electrical supply house, match up numbers to a new one.
Edit: OP says he has "lower voltage" going thru that heater, my experience is they blow and you get no voltage, anyone else find his problem odd?
Edit again: Yes, a heater is basically a fusible link, you just need to remove the element part, it has numbers on the tabs, do not remove whole plastic assembly.
Edit again: Did you put the green jumper in, or is that how you got it?
I put the the green jumper in to confirm that it would work without the fuseable link. What supply house do you recommend? Grainger? Allied...? Mouser?
Yes I find it odd but the wiring diagram is from 1963... It's antiquated for sure...
Grainger and McMaster should both carry them, barring any supply chain issues.... If you have an electrical supply house locally that carries Square D stuff they will most likely have them or can get them.