What should be straight? Is it the sawing or nailing that you think will be a problem? Most arches have probably often previously been a rectangular hole in the wall. Start by removing the arch. Cut the wallpaper with a razor blade beforehand, and maybe you'll save it if you’re not going to re-wallpaper. (It will probably be easier to get a good result if you do it in conjunction with new wallpaper/painting.) Otherwise, it's not a huge project, make sure to have a rectangular hole, saw three strips of, for example, MDF in the same thickness as the wall + 2-3 mm. Nail into the hole, spackle over and paint. ;-)