I have always use a tiny butane torch, designed for musical instrument repair, but now available in many hobby shops (Micro torch), Simply cover the track (if you want to work assembled) with anything not prone to damage.....saw blade, sheet of 1/8 in brass, etc. and heat until the area is warm enough to bend. Don mentioned a soldering iron, also very good. On the torch, just wave it back and forth to prevent burning one area , and again, keep in mind the damage you are looking for. If a driver backed into something, the fender will be buckled against or near the track or chassis. If he snagged the fender on a tree or building, it would generally be bent outwards. In any case, it causes a lot of extra work with a sledge...or just remove and toss it.

On the tiger in my photo, I copied the hits from a couple photos, but added a few more and hadn't gotten around to replacing all the HL markings and crew yet. Still, shows the idea.