The original is tin plated, I think. Might not be too far out to consider electroless tin plating -- this is, I think, usually a wipe-on formula, or it needs to be mixed from solutions beforehand.
*Checking*
Hmm, although, it may be that it can't be done on steel? I only see noble metals suggested... In that case, electroplating isn't much worse to be honest, but it is a whole setup you then need. (Power supply, leads and anode required. It can still be done in a wipe-on sort of way, hiding the anode in a brush soaked with electrolyte. The electrolyte should be shelf stable, no mixing required, aside from maybe a brightener additive that needs to be refreshed from time to time.)
As for coatings, don't put anything insulating between the shield and PCB; the rest of the surface is open season. Yeah, the waxes are probably not bad, they won't make it look any better of course, you'll still have a pitted surface (I assume it'll be some gray ratty pitted surface left behind the rust) but it will keep water out.
Be sure to wash and dry the metal very well; rust is catalyzed by water and salts, and the porous surface left behind can be very difficult indeed to clean and seal.
Guessing it doesn't need to last for too many more decades, so some of these options should hold until then.
Tim