I have a 50gal? (36"x12"x 28"high) the center brace was broken, much like what you are describing.
I figured I would give it one go and if it didn't work, just junk the tank or cut it up for parts....glass and such....
So, I took two pieces of plexiglass, the width of the tank from trim to trim, fashioned a bit of a "T" on either end, and one thinner piece (the width of the trim lip) and put it in between the two T pieces. I used acryllic glue and fused the three pieces of plexi together first, but i did not glue right to the T ends.
once the plexi t braces had dried, I slipped the ends over the tank trim, ad I drilled to small holes through the plexi T ends and through the tank lip.
I put to very small Stainless Steel screws through the T brace and the tank trim lip. Oh,,,, before I put the screws through, I squirted cement into the T ends, and quickly fixed them in place and then ran the screws through...
As Josh mentioned,,, unless you are somewhat passionate about saving this tank, it is probably not worth it....
I wanted to see what could be done with a brace so I played with materials I had on hand.
david