At this stage you should probably consider the ideal of welding a nut on. The heat traveling up the bolt should help to free it to some extent. The hex is already pretty well stripped I believe you mentioned. The weld needs to be pretty good and more than just a spot or two inside that nut. I would not cut the head off the bolt as the remaining stud may be really still seized pretty solid. You could also weld a nut on that stud as well I guess but the weld area will be smaller. Again the travelling heat is going to help a lot to loosen it in my opinion. Also go pretty hot when welding the nut on. You want a lot of heat transfer up that bolt plus of course it aids penetration of the weld. Some things just turn into a mysery it seems. The starter does need changed one way or another from your description.