The center seal is a compression seal. There are no rubber pieces to replace - unless one were to consider the rubber "o" rings around the outside a seal. The center seal does not seal pressure and a static pressure test will leak from one side of the engine to the other. Rather, because the impulses from the running engine are so fast, a series of grooves in the seal trap the compression wave and prevent massive leakage from one side to the other while the engine is running. Attached are two photos of the center seal - one showing the grooves on the inside of the outer piece of the seal and the other showing the inside and outside pieces of the seal.
Back to the "o" rings. Generally on the "o" rings, if they are missing, we just use a little gasket maker to replace what is not there.