Depending on what's left of the center brace, some people have had luck using glass and silicone to silicone the glass in between the front and back. Notice I said luck. This isn't something that I would personally do. The center brace doesn't just give somewhere convenient to place lights, but it gives structural stability to the tank keeping the front and back from bowing out to much.
A more permanent solution would be to replace the entire top trim, which would include a new center brace. This would be a long job, and a pain to do, but IMHO would be the better solution if you want to use it as a fish tank.
I would also just make sure the tank is made for aquarium use. I know some reptile tanks have the same dimensions, but the glass used isn't as thick and won't be able to hold up under the pressure of a tank full of water. These also sometimes don't have the center brace. On my 75g the front and back panels of glass are close to .5" .