Welding will be a difficult task, as the aluminium will have absorbed some oil, and will need to be thoroughly cleaned out before a good weld can be performed.
This usually involves soaking in degreaser and/or running an oxy torch over the area to be welded.
I have had reasonable success with Knead It, but again: cleanliness is key. To do a good job, you will want to remove the sump, thoroughly degrease inside and out, and then thoroughly clean with brake cleaner or similar.
If you leave the sump on the engine, then you will need to work hard at ensuring that oil isn’t weeping through the crack while you are applying the putty and it is hardening. I have done it a few times, usually by flushing the inside of the sump with copious quantities of brake cleaner, but the results have varied from “excellent “ to “mediocre”.
At least your crack is not catastrophic, so if the repair fails it is only going to weep again (rather than dump the entire contents of the sump onto the road in three seconds).
