Try "Krud-Kutter" stocked at Home Depot. It's a paint remover and cleaner (I clean my dirty fingerprints off kitchen cabinet doors with it). As always, try any unfamiliar product on a similar surface, or any surface, just to get the feel. Mask off the plate, or (as I see in your other posts it's an early car) remove it and try cleaning it off the car. You can get new riveits at AMK products. You can also get a new data plate at Marti Auto.
Jim