Has the wing center section been sheeted yet? If not I would probably consider cutting the root rib and ply rib out, and making a new root rib. It would not have to be absolutely perfect since it will be sheeted over anyway. Just remember to set the dihedral angle.

If its already sheeted then I would try the razor saw technique, then sand with coarse sandpaper- on a long sanding block to make sure everything stays straight. Slow but probably do-able. Luckily any imperfections that result will not show once you join the wings together and finish the seam!