Any decent shop that sells tanks can order up a new top frame, this includes an unbroken center brace. To get the top frame off heat it up a bit with a heat gun, and use a razor knife to slice the now warm silicone. Don't worry about breaking the old top frame, you're going to throw it out.
Once it's removed, clean off the rest of the old silicone, dry fit the new top frame to make sure it's the proper size. If it is, add a bead of silicone inside the top frame, push it securely on & let it dry for a day before filling.