I will attempt to give it a try. (And this is what I would do, some I am sure would argue). 1. If the stud is broken off where you can grab it with vise grips, I would soak the stud with a good penetrant like PB Blaster for an hour or more. Then I would mildly heat the fender extension with a plumbers torch. It is pot metal so take care not to get it "Red Hot" You are trying to expand the threaded hole to where you can work the stud loose with your vise grip pliers. With patience, you should be able to get the stud moving. Start by going back and forth, you need to "grind away" any loaded up gunk on the threads by moving it back and forth. Once moving, you can spray more PB Blaster on it. The Penetrant helps break down the corrosion on the threads. You may also have to heat some more, but I would try not to use heat after you get the stud moving. 2. If the stud is broken flush, spray with PB Blaster, center punch the drill into the stud. If you have Left hand drill bits use them, if not use regular drill bits. Start out with a small drill bit and step up. You want to try and keep the bit Plumb or straight while drilling. Going in an angle will go through the side of the threaded hole and possibly ruin the threads. Once you have a hole drilled in the broken stud, use a screw extractor or "Easy Out" as they are sometimes called. You still will have to use heat to get the stud moving, again once you get it moving, back off the heat and use the penetrating oil. If you attempt this, keep in mind that it is a process that requires patience as you can ruin the threads et. in the fender extension. However with luck, finesse and patience it may work for you. Do not use an Acetyline torch as they get too hot.Also keep in mind that the threads if damaged can be re tapped, or you could JB weld the studs into the Pot metal just as you could the Fiberglass. The piece is already unusable so it is worth a try to save it. You have nothing to lose. Hope this gives you an Idea or approach to the problem.
