The head cover on BW II is mag. I think maybe the stator, ignition and clutch cover are also, but I think the pan is aluminum. Just saw one on Fleabay that had the drain plug relocated to a less vulnerable position (probably had the original broken off) And the welds definitely looked like aluminum. I assume 12Rs are similar. I remember when they came out, early blurbs said some engine covers were mag for weight reduction.
Nice piece of kit, BTW.
It may be a pig, but it's a pig with a rocket in it's ass!