not sure if this would fit your needs or not, but it may at least hold you over until you track down an elusive original mounting plate...

i have seen folks take a non-empire cartridge mount (like zb-10 in the picuture) and cut it down to work quite well in the Empire tonearms. just cut some off the end and drill a little hole in the middle.

considering how rare the original mounting plates are (i assume you saw the crazy prices being fetched on ebay), this may at least work as an interim solution.