AL Welding & Crack repair
Aluminium is best welded with TIG using an AC / DC rectifier set to weld on AC.
This is seriously important as DC will destroy the panier. Extra plates could be welded (tacked in using MMA /Sitck welding) if the proper consumable is used. Take care to remove the slag from this process, as it will corrode the metal.
MIG again can be used, with no serious damage.
Get the plates tacked in this will increase the rigidity.
Get the guy to demonstrate his skill prior to AL welding.
The exhaust fracture, TIG, MIG, NOT MMA or stick, it will burn through and leave you in more trouble than you went in with. Let me know how you fare, I'm a welding engineer, Cheers & Good luck