I don't know about the legality of it, but maybe use the good (better?) plate as a cast to make a pair of 'glass or resin plates? Is there a law that the plates HAVE to be metal? if so, maybe do the above and place the one good one in the rear window or some other visable, protected place so they can see that you really DO have the real thing.. As long as they are decent copies I don't think anyone would bother trying to feel them to see if they are metal....
what I am htinking is just lay some 'glass sheet/mat over the good/better plate and resin them to basically make a female (inverse, or negative?) mould, then coat it with something the resin can't stick to and pour resin (or a lot of 2-part epoxy?

) into the new mould? or maybe use that mould to shape some styrofoam (making a male mould, which would look just like the original), then lay more 'glass on the 'foam. that should end up with a copy.
you may lose some definition, but you can work that out with a little sanding and filing I would think.
am i completely off base here or would that be possible? (though a ton of work...)