You should be able to get a new circlip at about any well stocked auto or hardware store. You want one for an interior retainer rather than one used for exterior. Get one with the two tabs that stick up rather than the thin kind with holes in the end ears, they’re easier to remove down the road if you need to. As for the rubber cap, there should be two kinds with the kit. A full rubber cap and a disc. The disc is retained by the beat up metal piece you have, although you should be able to use either the disc and metal retainer or just the full rubber cap. You don’t use the metal bit with the full cap.