If they're rounded out, get the next larger size (sae or metric) that you can just manage to cram into it. Then mix up some jb-weld, glop it on the end of the allen key, then hammer it in and let sit overnight. Ensure there is clearance to rotate it all the way around. If not, do it with a 3/8" drive allen, or as mentioned sometimes finding the right size torx bit works too.
Another trick if you have an impact screwdriver is to use a dremel to grind a slot in the top of the bolt, then use the extra wide flat bit and a mini-sledge to try and crack them loose.
All else fails use needle nose vise grips and use a good torch to heat the area surrounding the bolt, not the bolt itself. Regular propane won't cut it, MAPP is better if you don't have an oxygen torch. Obviously wear gloves and work quickly.
Another trick if you have an impact screwdriver is to use a dremel to grind a slot in the top of the bolt, then use the extra wide flat bit and a mini-sledge to try and crack them loose.
All else fails use needle nose vise grips and use a good torch to heat the area surrounding the bolt, not the bolt itself. Regular propane won't cut it, MAPP is better if you don't have an oxygen torch. Obviously wear gloves and work quickly.
