Yes the pitting and cracking is more common on a mag deck than an aluminum deck. Magnesium can be welded using an Heliarc welder in A.C. mode with pure magnesium rods. I have had a couple of cracks welded on a 1966 restore back in 2012 and it turned out well. Those mag decks just seem to develop cracks out of no where, in areas that you wouldn't even think were stress points. The big problem these days is the price of pure magnesium welding rods. The welder that did my work says it cost him 90 bucks a pound, so if I want anymore mag welding done I will have to supply the rods myself.
As far as the pitting goes, I think what works really great is a product called Lab Metal (high temp version). If you clean the metal good, media blasting preferred, then you can apply the Lab Metal over the pits, sand it relatively smooth, then take it to the powder coat shop to get it baked and powder coated and it will be almost as good as new. I think some enterprising young person who has the right equipment, knowledge and lives where there are plenty of old mag decks laying around could make good money refurbishing those decks in this manner and selling on ebay. I think someone has already been doing that with the newer aluminum decks.