You will need to remove the covering from the affected area. Clean the are well first to remove any fuel/oil. Then cut away the covering to a point approx 1/4 inch short of the first GOOD rib and fore/aft to the leading edge and rear spar/trainling edge. Oh, before you do this try to iron the covering down tight to the edges you will be cutting near. The ribs that are merely cracked might be repaired by pushing the pieces back into place and addind a few drops of thin CA to rebond the structure. Be sure the rib is still well attached to the leading/trailing edges and spars too.
For any destroyed ribs, use an existing rib that is in good condition to make a pattern (paper is good) and transfer this to correct size balsa for a new rib. This can be attached to existing pieces or totally replace an old rib. Again, anchor at all appropriate points to structure. Sand for a smooth surface contour and recover. You should have both an iron and a heat gun.

Hard to tell much better without a picture of actual damage.