I'm not understanding the term "fill plunger." Do you mean the ball (or flapper) that opens and closes the flush valve, or is it something related to the fill valve (with the ball for a float) or the overflow tube?
This looks like a one-piece toilet. I know that some silent flush systems (which this one appears to have, based on the large inlet opening on the left of the bowl below the water line) use china fill valves that are pretty much NLA. If the fill valve needs to be replaced, the flush action might still be silent, but the filling won't be.
If there's a problem with the flush valve not sealing any more, there are kits available to fix that with a thick sticky ring that gets pressed onto the metal valve seat and a new flapper mechanism that presses onto the top surface of the ring. I used this on a 1947 American Standard toilet at our previous home and it effected a successful, permanent repair.
If the fill valve can't be fixed, the Fill Pro anti-siphon type with no float is the way to go. Fairly quiet and trouble free, with easy adjustment for tank fill level and it takes up a lot less space in the tank than a Fluidmaster or other traditional type. See link.
It always used to be that plastic was a cheap substitute for brass tank hardware. It's tough to say which would be better for a replacement cartridge. It seems odd that plastic would cost more than brass/metal. Others here may know which is the better choice.
It's very nice of you to be helping out with these repairs and trying to save household appliances and fixtures that are worth keeping.
FILL PRO ANTI-SIPHON FILL VALVE
www.diydirectsupply.com