I have some damage to my engine casing as pictured, that could do with repair. I know the outer covers are magnesium, but not sure about the engine casings themselves? 

Use a coarse file to create some filings onto a piece of paper, set the piece of paper on fire and use it (the non-burning end) to sprinkle the filings into the flame.
If they catch fire as they meet the flame its magnesium otherwise its aluminium.