You can also order the top frame and replace it completely. The tank probably wont fail with the center brace broken on a 55 but you shouldn't take the chance.
You are going to have to take about half the water out of the tank and repair the brace. I had something similar happen to a 55 of mine.
Here is the solution I came up with.
I used a pipe clamp to gently pull the center back together. Be VERY VERY careful doing this. the PVC
was cut a bit short, one elbow was glued into the length and then I put glue on the other end and in the other elbow, when I go the center back where it belonged I tightened the clamp down a little more and put the other elbow on. After an hour of letting the glue set I removed the clamp and everything was good to go. This held for over a year before I took the tank down.
My vote though is to replace the plastic trim completely