Marineland used to have issues with this exact type of damage IIRC. You can drain the tank, remove and replace the top bracing. Additional support can also be provided as a short term fix, I took a clamp to my 55G when the center brace snapped.
Was trying to avoid draining the tank and replacing the brace. Looks as if I have to. Was thinking of using some sort of epoxy and plastic to repair it.
I have used both J-B Weld Plasticweld and original J-B Weld on several tanks over the years. Many times without draining. Using a lot of it works best all at one time. Sets up really fast and dries very hard.