Magnesium or Aluminum?

Imagine my dismay as I remove the oil drain plate (bolts came out nice I think to myself).  Peel back the gasket and voilà, WHAT THE HELL HAPPENED!?!

I initially thought that the block was cast aluminum, but I think now it is magnesium, I haven't found anything on-line to tell me what it is exactly but I've never seen aluminum do this.  Magnesium however will corrode and dissolve into nothing.  It is a sacrificial metal on pontoon planes that is put on strictly to corrode so the aluminum doesn't.

Anyway, there was that and this:

This picture is of my 28 PICT carb, I had just resealed it and...I'm missing this very important piece off of the carb top.  Can't buy it separately, a whole carb isn't cheap and hard to find (not many people stick to the original 1200cc 40hp engines of the 60's).  I did however find a person on www.thesamba.com who was selling carb tops!  One small hurdle is overcome.

Now back to the block, well I did get two engines in this deal, except I thought the other was a 36hp 1200cc (which is designed differently) but upon furhter investigation it is what looks like a 40hp and the oil drain is a perfectly machine surface like it should be.  It is in rougher shape, some pitting on the outer cast and so on, but overall it's usable.

Oh, and before I forget, I was removing my old heads of the "bad" engine block and the head bolts were seized extra specially tight so once I cut the bolts to remove the head, and now I'm hammering the head bolt out..."CRACK" broke off a fin, to close to the bolt hole so much that I don't feel comfortable with it.  I was originally told these heads were the "new" style square boss, but again upon investigation I find out that they are the lesser round boss style which are prone to valve seizure when running hot, so no real big loss.  I was able to get a pair of used square boss heads off of www.thesamba.com for $15/ea plus shipping.  Not bad, that is until I had to look at the "older" thought to be 36hp engine and I instantly realized that the square boss heads are on there...I'm thinking this engine as old and rough as it looks has been rebuilt at least once before and I probably should have started with it.

Oh well, I AM learning.

jd