You need a galvanizer, not a plater. Most platers don't galvanize.
Technically, galvanizing is not plating as plating refers to an electro-chemical process. Galvanizing is a "hot dip" into the zinc alloy. I believe the spark plug shields were stamped from galvanized sheet. That means the edges are raw steel, but since zinc has a tendency to heal itself, the edges don't rust right away. Pretty tought to duplicate the original spangled galvanized coating by regalvanizing the finished part. Galvanized usually has a very bright appearance when new and gradually dulls and grays with exposure to the atmosphere and its' self-sacrificing nature.
Zinc plating is, however, a plating (electo-chemical) process and does not have the spangled appearance. Big difference.
Dave