Is the main dealer still going? You could ask if there plates are still the same style. If you like the original plates maybe a numberplate comapny (one of those ones who do non-street-legal types) could scan in your old plates and work from there. Failing that, you could buy a new set and do as i do, retain the originals in the garage on the wall! I always fix my numberplates without screws as it looks a lot cleaner and you don't have the problem your now faced with with the rusty screw.