I had an old, cracked sump in my garage from the first time I broke one, the ideal subject for learning to MIG weld ally
To convert my MIG from steel I needed a spool of aluminum welding wire and a bottle of Argon gas (Halfords sell both; £9 for the gas, £6 for the wire). I was also advised to replace the wire feed tube with PTFE and go one size higher on the tip (1mm up from 0.8mm). In the end, both of these proved unnecessary.
Welding Al is very different to welding steel. Al has a low melting point but conducts heat very well, so needs lots of current (to maintain heat in the workpiece) and a high feed rate (to prevent the wire melting back to the tip).
I first ran a bead over the cracks, making the sump oil-tight again, then cut and welded on a piece of 3mm treadplate :
The welds aren't particularly pretty (TIG is best for welding Al), but they are strong.
On Wednesday I should be getting a sump gasket among other bits, so I'll see about fitting it then.