Brace is there for a reason, it is structural piece keeping your tank from bowing and seams from coming apart. Once that brace cracked, its integrity is gone. It is injection mold, so once it's gone, it cannot be repaired anywhere close to the way it was originally made. With all that said, what you could do is remove whole top plastic frame, and then do a proper euro bracing. That way you have strong brace that will keep your tank safe, and get rid of the plastic brace that is now almost useless. You could also look into buying complete replacement frame, but that can be more work than just getting glass pieces and euro-bracing.