Tig welding aluminum is fairly simple. Some complex welders are better at cleaning. My welder is just a square wave tig and does not have the wave tuning options. Therefore the inside of the pan needs to be clean also.

Done this before. Drain the oil. If the crack is large enough spray solvent into the crack. Acetone is best. Leave the plug out and remove breathers or even the valve covers so the engine can vent. If everything is clean, easy breazy.