You might check with some chrome plating shops - they deal with pitted Zamak all the time when re-plating old classic car stuff, so they know how to deal with it. I believe they grind out the pits with a dremel tool, fill with solder, and then grind smooth. It ain't cheap though!
Carla's SS SeaCraft Classic logos at
Moesly SeaCraft look nice and would probably be cheaper!