if you know somebody who has a tig welder or even a wire feed welder(gas shielded because you need a good weld)
what you want to do is put a nut down over it and weld the nut to it
as the weld is cooling use a propane torch to carefully spread the heat into the area
then when you get the aluminum up to 250-300 degrees or so by then the weld will be cooled enough and it should wrench right out nicely
if that fails then use the upper case as a drill guide to get whatever drillbit fits the upper case started on a good dead center ,and just enough to create an accurate full center cut,then follow it with the tap size drill,or one size under that
having the upper case in place will help you keep it straight
you will need longer drill bits than standard
once you get into the broke off piece 8mm deep or so switch over to a left hand twist bit
by then you can take the upper case off as well
