Ok now that I am back inside and not on phone here’s what I would do. If you can, finish cutting through the lap joint if not it can be done without cuts they will probably break at the joint anyway. Take a 2x4 the entire length of warped board, drill holes into the new board horizontally, larger than that of your screws diameter but smaller than the head of the screw. These screws will need to be 4&1/2 to 5” in length. It might help to have small washers for the screws as well. If you have c clamps or longer, clamp the new board to the warped board and get it to move as much as possible the screws will do the rest of the work for you. How many screws will depend on the strength of the existing board which shouldn’t give too much resistance. Lag screws would work as well or bolts that are long enough.
Do this from inside the cage