I'm no expert by any means, but I would soak the brass in vinegar and then use fine steel wool on it--lightly.
The steel--wash it with hot soap and water and a nylon bristle brush. Dry it, then put very light oil on it for a day or two. Then use steel wool on the back and a razor blade scraper on the plate. See where that gets you.
It will take time, so don't get in a hurry.
But I'm sure there are guys on here that know more than I do.
What say you, fellows?