If the bolts are rusty to the point where you can't remove the nut then I suggest you try to remove the remaining bolt carefully without cracking the porcelain. If you can do that, they sell new johnny bolts at most hardware stores that screw into the floor on one end and have threads on the other for a nut. Be careful removing the remaining rusty bolt because it doesn't take much pressure to crack the porcelain. If you do then it new toilet time.
