Ok, now it's more clear...
Search for Aluminium bi-component toothpaste (I hope to be clear, no other word in italian), prepare it and screw the plug, with it on, onto the case (not so strong, just put it in) and after 2-3 hours add some other (new) aluminium paste between the plug an the case... (it's the same procedure with cooling repair kit, maybe it's easier to found and also working well)
Had worked in the past, it will work again now ;-)
More hard job is to weld directly the plug, with diecast tools (electrodes in nikel, or electrodes for welding aluminum) , onto the case...
If you go where people repair trucks, normally you'll find out all the necessary
-> I agree, never use helicoil.... pls!