Oil pressure relief piston - plastic

[Tonger]

While removing the piston from the bottom of the pressure relief valve on my oil console, we cracked and lost the plastic that is held in by a screw. Not wanting to purchase another one for $120 or so, I was wondering if it might be possible to replace the broken plastic. It should be simple enough to cut a circle out of a plastic sheet or even 3D print one.

(Why is this plastic?)

My question is, why is this plastic and what kind should it's replacement be? ABS? Replace with aluminum?

Do I even need to replace it? Spacewise, the missing piece doesn't cover much space.

Thanks!