This happened on one of the large Sukhois recently during an "awkward" landing. The trick here is to get as much of the original structure back in place as flat as possible. The fit will not have to be perfect, but flat and in the right places will be important for the next part.

After all the flat work is done locate either some 8 ounce fiberglass cloth, or better still, 9 to 12 ounce carbon fiber cloth. All the work with the composite material will for the most part take place inside the tunnel. Be prepared to get a little frustrated with working in tight spaces. Carbon is a lot more work but much stronger.

Cut strips of cloth that will fit pretty close to the size of the sides and deck. Lay them up using a laminating epoxy. You want to apply the fiberglass dry and press the epoxy through the cloth. You have to apply carbon wet. Try to pull as much excess epoxy as possible out of the cloth using a squeegee of some type and a paper towel to wipe away the excess after the "pull". Get either of the composite materials to adhere as tight and as flat as possible to the base wood.

Let it all dry, and cut some 1" wide strips of glass or carbon that will be used to reinforce and tie in the corners of the top and sides. After that dries laminate the top of the plate inside the fuselage with some light weight glass cloth to finish tying the structure together.

You can do the same thing as above using ply plates cut to fit as a "strong back" or doubler but it's comes out a lot heavier and not nearly as strong as the composite methods. Any way you go about it is a lot of time and effort but you save the plane.