Can you take any pictures and post them here? What engine are you running? What kind of damage is there?
It's kind of wide open as to possible causes, but I'd check to make sure your clutch bell and spur gear are meshing properly. Sometimes after a crash, the engine may move a bit and the CB and spur may not be meshing smoothly.
Also, may check that the brake isn't dragging. Check all your linkages, make sure nothing is bent. Additionally, give your drive train a complete once-over, make sure your dog bones aren't binding.
If you can possibly throw a couple pictures up, that'd help tremendously.