New plastic parts are available from Swinks shop on Shapeways. To use the plastic parts with the new base you'll have to drill the rod and ring holes in the base oversized for the plastic rods. This would be my preferred option since the plastic parts have less mass and snappier performance.

https://www.shapeways.com/shops/dutch-orange

I don't know about just replacing the plastic parts with metal. The plunger and stop bracket may not be compatible, you may need to buy these parts as well, but I don't know, I've never done this conversion.

If you convert it the plastic parts to new metal ring/rods you’ll need the metal and fiber yokes. They’ll work just the same. The plastic yokes are shaped to fit the plastic rods, so they’re incompatible with the more common metal ones.

I’ll be the first to throw my hat in the ring to buy the used plastic ring/rod and yokes from you however!

I'd keep the original plastic parts if they are still in good condition. As mentioned above they are nice and snappy.

As long as there's no wear/play, keep the original plastic. They don't need replacing as bad as the metal

Here is a better link for just the pop bumper parts I helped people with:

https://www.shapeways.com/marketplace?q=swinkscsbumper&sort=newest