I don't know an easy way to fix this, but a way that works well involves a wing jig. It's probably too late to fix this particular wing with a Great Planes style jig, although using it from the outset would have avoided the problem.

If I had your wing in my shop, I would put it on my Aeroalignment Jig http://www.aero95thsquadron.com which would allow me to brace the leading edge and trailing edge to straight alignment. Then I would put truss ribs between main spar and trailing edge. Glue them in, and when the wing comes out of the jig it will be straight and stay straight.

Making a pattern for your truss ribs involves stretching your rib profile to the diagonal distance across the rib bay from the TE corner to the opposite main spar corner. This is a pencil & paper exercise. Having a drafting board makes it a bit easier, or gridded chart paper if a drafting board is not available. Keep the height of the truss the same at either end, keep the relative vertical position of TE end and MS end the same, and fatten the rest of the profile just a bit by eyeball. Glue them in while the wing is braced in the jig, then use a sanding bar to bring the profile of the truss ribs down to the same surface level as the kit ribs.

If you don't want to invest in a great jig, then figure out any method of bracing your wing straight while you truss--with clamps or whatever.

It would probably be easier to cut a new set of ribs from the other wing half, buy some new spar stock and build the wing over again--preferably on a jig. As a final note, trussing works better when they are built in from the beginning rather than as an afterfix.