Magnesium is a very brittle metal. In the 50's it was the wonder-metal of the time. Was outlawed in the 60's for use in lawnmower decks, too many people got hurt when rocks went through the housings, killing/maiming people. Personally, I wouldn't use them.
Aluminum rockers won't have the strength or hardness of a steel rocker. Add a little over 50% more material to get the strength of steel, but you can't come close to the hardness needed for the pounding on the valve stems. Okay to run, but definitely not a long-life product.
