Hello Al and welcome!
Being Austrian, unfortunately I can't help you with the legal issues.
However, I have fitted a home-made "number" plate to my A10. I posted two pics of it, maybe they can be some inspiration for you. I cut it from 2mm sheet metal, welded two bolts on and had the whole thing spray painted. The lettering was sprayed on in gold paint, I had the templates made from self-adhesive masking tape on a cutting plotter.
To disarm it a bit, I fitted a rubber beading around the edge. So far, the authorities here did not complain about it.
Like always, its a matter of taste. I like the old-fashioned look on the touring A10, and it saves you a lot of questions like "How old is it, mister?" or "What bike is that, mister?"

However, I did not fit one to the Shooting Star, as I feel it would disturb the sporty lines of the sports mudguard, but like I said, that's a matter of personal taste.
Cheers, Markus
P.S.: As spring is coming, more pics of my bikes to follow, like I promised when joining the forum during winter. Provided its gonna stop raining some time....