FF31,
Depending on the type of distortion, you may be able to bend the bumper back into shape by heating it. Try very hot water, that is near the boiling point, and dip the part into the water for several seconds. Then gently apply pressure to the part so as to straighten it. The procedure requires patience and trial and error, hopefully less error.
Try a piece of spru from the same kit so that you are dealing with the same plastic compound and it should have the same characteristics as the kit part. The spru will also give you an idea as to how long to heat the part so it will "bend" into shape.
Reshape the part a little at a time and hold the part until it cools so it will retain the new shape. Try this, it works. I have used it to straighten out bodies that are twisted, as well as warped aircraft wings and fuselages.
Remember to take your time and be patient. Also remember that the plastic in two different kits, even if the manufacturer is the same, can be very different. Harder plastic takes longer to soften and seems to have more "memory" than the softer plastic. Experiment before you try the real thing. Good luck.