Radio cases were and still are made from Galvanneal, which is cold rolled steel sheet that has been hot dipped in zinc, blown off with 'air knives' for zinc thickness control, followed by annealing, "causing iron and zinc layers to diffuse into one another causing the formation of zinc-iron alloy layers at the surface. The annealing step is performed with the strip still hot after the galvanizing step, with the zinc still liquid. The galvanising bath contains slightly over 0.1% aluminium, added to form a layer bonding between the iron and coated zinc." (From Wikipedia.)
Since Galvanneal is a specialized steel mill product, there is no way to do a restoration re-plate in an identical way. The surface can be renewed somewhat by wiping or scrubbing with a Scotch-Brite pad and vinegar followed by waxing.
A vintage radio case could certainly be zinc plated, but it probably won't look quite the same as Galvanneal. Besides, there were stickers and stamps on the original cases.
