Grind down the remaining piece of the stud to a flat surface, then, drill a hole the size of the replacement stud into the center of the old stud. Use a punch to mark the center of the old stud and use a small bit first then use larger bit sizes until you get to the proper hole size. After this is complete, get a piece of threaded rod the size you require and tap the hole out to the proper thread size. Use panel adhesive on the threads and screw into the hole. Done!!
I used this method on the Di cast hood louvers for my 68 Cuda. Worked great! Just take your time and go slow.