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.