uh ohhhh

durafix/ alubraze/ easyweld/ aluminium easymelt easy braze rods

you clean the area with a stainless steel brush, that should only be used for this job, the longer you brush the easier it flows. you want every remnant of aluminium oxide out of the way
you can't do this with brass or normal wire brush has to be new clean stainless brush

you heat the aluminium until it burns off sharpie ink and you feed your rod in like gas welding

used it on cracks in none load bearing sections of motorcycle casings to stop the oil getting out

works

you are basically gas soldering with aluminium ish "Monkey Metal/Pot Metal" rods

need a small tip propane torch 1/4 inch with good control, and you need to be aware that aluminium turns liquid in the middle while holding its external shape and then collapses so be careful with the heat you don't want to go that far.
do it with the drain plug in, less chance of it going WOOF in your face
I'd just fill the sump with mig welding gas and cap everything off. its a heavy gas but will rise when hot