Here's a real simple way to do it. If you have a picture of this that you can trace in the same scale (most printers will allow size adjustment)then cut out the lettering, and glue it with 3M spray adhesive. Attach the letters to styrene sheet about 0.20" thickness. Cut them out very carefully. Then align them so that they are in the same width from each other as the real deal on another piece of sheet styrene. Apply some sculpy putty to it. DO NOT BAKE IT!! Instead, let it sit for a few minutes to harden up to the styrene sheet. Or, you can put it in silicone rubber to make a mould of these letters. Remove the styrene letters. NOW - pour in some polyurethane resin from whereever you get yours from. I get mine from www.dascarplastics.com
let this sit in the putty/silicone until it cures. You can remove it aftewards, then cut a hole in the bumper for the new letterhead. Sand around the edge to hide it once it's installed. This is just one of many suggestions, but I hope that it will be useful at helping you at some stage in time. Maybe this isn't the shortest, but maybe the easiest way to do it.