Doing about 2 minutes of research reveals the following:

Early aircooled engines (25 & 36-40 hp) used AS41 alloy for the cases.
AS41 is: aluminum 4.2% silicon 1%. The remainder is... magnesium

Later 1300-1600 motors used AS21 alloy.  AS21 is:
2.3% aluminum, 1% silicon a bit of zinc and manganese and, you guessed
it. Magnesium.

<http://www.northerndiecast.com/mcalloy.htm>

The type IV and wasserboxer case material seem to be a bit more of a
secret. But if someone can come up with the spec, the composition is
just a minute away.

There's also a bit of info on the type2.com website.

Jim