I have used a mill-bastard file before to dress the edges on the standard KO cutters, but if yours are totally flattened you made need to look into professional sharpening or else maybe just replace the KO cutter itself, as the 2" isn't that expensive.
I read somewhere that some guys sharpen theirs with a belt sander. That sounds like it may be a little crude, but if yours is really that bad it may not hurt to try.
