Pretty sure it's aluminium alloy. Whenever I need something like this done I go to an engineering/welding workshop (found in industrial areas of most towns) and they charge $20 or $30 for the job.
Yes , as a BMW I am pretty sure its a cast aluminum alloy , probably not of the best quality, but a workshop with a good AC high frequency welding machine and compitent welder should be able to do it. , more like $ 100 tho it would take a bit of time, about 1 hour to set up, weld and clean.
You will have to strip it first as the heat will damage the plastic shims if the break is any where near them.
Brian.
good luck ,,hope you get your mount fixed ,,I've had work knocked back in cast because the shop thought the cast quality was so poor they wouldn't even try ,, and other stuff done without a problem it wasn't always cheap though as mentioned the cleaning and set up can soak up a 100 or so pretty easily.
If you decide that its to much trouble to fix , I will take it off your hands as is f, or the cost of post to me here in WA and as it happens we do have a nice AC , HF , TIG machine here at work and I am the competent welder .
HF = high frequency , its the only way to weld aluminum.
PM me if this is the route you take , I might as well try ? , I will post the results , with photos if anyone is interested .
If you decide that its to much trouble to fix , I will take it off your hands as is f, or the cost of post to me here in WA and as it happens we do have a nice AC , HF , TIG machine here at work and I am the competent welder 
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