Unless someone can firmly identify the material, you should steer clear of this. If you want to do the job, get a small (nickel sized) sample of material, and for about $100 you can have the material identified.
Once you know the base metal, you will be in a much better position to do the repair.
My two cents.
Charles
A not so precise means of identifying aluminum, magnesium and zinc die cast is to strike an arc on the metal with the GTAW process. If the arc is basically white it is aluminum, if the arc is green it is magnesium and if the arc is purple (violet?) it is zinc die cast. If it is zinc die cast use the alladin 3 in one wire from I believe Allstate. If it is magnesium and it doesn't have metal flakes, chips or powder it is safe to weld without danger of fire.
If you want to do something very stupid but it will make a very hot and bright beach lamp. Set a magnesium vw block on the sand and use a road flare to set it on fire. Bring your sunglasses because it is going to be bright!
Guessing isn't the way to go about this as you know. I'm guessing as well when I say that I "think" you may be right about the block being magnesium. Old opposing VW 4's were and you had a really hot fire when they caught fire on the side of the road. Dizzy and fuel pump were mere inches apart=fire.
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