Honestly I would just replace the whole tank. Not sure you will find another frame. I think petco covers the 55 with their dollar per gallon sale but not positive.
I had a tank about that size when I was a kid, with out top frame, they just to be aluminum frames, the silicon on the sides expanded and aloud the front panel to separate some enough to let water come out, it was dangerous, can't imagine that again in this days, unfortunately I was a poor kid supporting my own addiction to the freshwater hobby.
Just a safety call.