[ithacaartist]

One might take John's approach one step further. You will indeed need to reduce the radius of the business end of of this little stretcher/jack.. In order to persuade the material to adopt the proper radius, one must bend it slightly beyond its intended profile--this allows for a certain amount of "spring-back" which will occur. However, leave the other half of the jack at a radius that will fit as closely as possible the radius of the undamaged portion of the guard. The jack will obviously push against two opposing portions of the curve inside, and an exact radius will guarantee no deformation on the opposing side as the operation is carried out. You may also need to adjust the radius of the working end to achieve the desired round profile smoothly.