I have seen replacement frames for rectangular tanks, not sure if they are available for bowfronts.
If you want to try and fix it, the idea of laminating the brace is a good one, assuming the break is not right at the side frame, though I would go with epoxy over silicone. I believe the frame is made of ABS plastic, I would get some ABS sheet (local plastics supplier or online store like Mc Master CARR) and solvent weld the reinforcements in place. If done correctly it will be stronger than new.
Gary