what you'll have to do is get a clean model file and copy and paste the polygon into the edited one.
for example, say Polygon0 has been deleted from Fox. get an unedited FitFox.pac (or .pcs), export the model, open the model in a hex editor, go to Polygon0, copy all of it (you don't need the header, but it shouldn't matter). then open the edited Fox model in the hex editor, go to Polygon0, highlight the entire polygon (or just after the header, whichever you copied, it just needs to be the same length), then paste it in and save.
Only question how to found the polygon on the model?