You could clean out the cracks and replace with treated lumber. Buy them from a full service lumber yard that has a planer that will plane them for a buck or two per board. Fit and cut them for lenghth (marking the location on the side of the board) and measure for thickness to plane. Then have them planed to fit. Remember a treated 2by4 usually measures thicker than untreated 2bys of 1.5", with treated about 1 5/8" thick.