Had the same thing happen to me. was able to get some vice grips on the exposed part of the rusted bolt that remained 'inside' the bowl. Applied PB Blaster several times and let it sit and work, then took a propane torch to it heating up the bowl area where the bolt is. used the vise grips to move the bolt back and forth with short movements to losen it. just takes some patience but you should be able to get it out without drilling. Good Luck!