I've had this problem too-- only the builder was me-- not the manufacturer.

You can go into the formers at the rear of the plane and cut balsa cross-braces to shim it back straight.

What you will wind up with is an "X" brace.

Cut and sand the angles on one brace. Glue it in place with CA. Now cut and sand the next brace the same size. Sometimes you have to make it a little longer than the first brace.

You can wedge the brace in and "tweak" the fuselage back straight. Clamp the two braces when you get it straight and glue it while clamped.

It's a pain in the rear- but it works. And when your done--you have a place to glue any pushrods that might run past the braces.

Hope this helps.