The old engine cases were aluminum and magnesium. The transmission cases were all magnesium. And you are right, either one on fire is very, very bright ! Will blind ya ! The fumes can kill ya, and if ya try to throw water on it, blam !!
Burning Magnesium is very dangerous, the only way to put it out is to cut off the oxygen to it. Just like the Magnesium and Phosphorous bombs used in Vietnam .
The bug motor cases were all magnesium-I think the heads were aluminum and maybe some other parts. The later bus with the Type IV motor ('Porsche engine" as some folks say) were aluminum, engine and transaxle. Technically the 'transmission' would be a transaxle. . he he he. . . (sorry, had to) . and yeah, the transaxle was magnesium on the bug. I did old VWs for many, many moons. ... traveled all over the country in a '73 Westy and make money sporadically, tuning them up, still to this day.
Also, the sodium filled valves as found on the Porsche 914 were 'fun' if broken. . LOL (same type engine as in later bus but with some differences like the valves) sodium go *BOOM* (big flare up - also DON'T TRY TO PUT OUT WITH WATER!)