You can try to weld a nut onto the stud and then use a wrench to take it out while its still hot. Try to just weld the nut, don't allow the aluminum to get to hot it can warp. Keep most of your heat on the Nut your welding and the stud.
Heat the stud. let it heat soak into the motor. Dont heat the aluminum it self. Should use an oxy actlyn set up. Get stud dull red hot not bright red. Let red drop away and then try again. Weld the nut if it still slips. Don't snap it off or you will need to get very good at drilling.