Lawnmower deck aluminium cracked. How to fix it.

Here are links to a couple of firms who offer aluminium welding:
http://www.themetalman.ie/
http://weldandfab.ie/

If you wanted to do it DIY you would need an electric drill, a pop riveter and an offcut of thin aluminium sheet, the latter being the most problematic unless you know someone who squirrels away all sorts of potentially useful bits and pieces.

hammer it back in place as best you can and then use a fibre glass kit

sand it, paint over and bob's your uncle