Three simple options:
Recall the brace is only to lend structural integrity and keep the back and front panels from blowing out. That being said,
1) Drill one or two or a few small holes on each side of the break at least 1/2" back from the break. Use heavy wire to bind it back together or like stitching - just make sure the wire is really tight because it will assume the pressure that was formerly on the bracing;
2) Sister it - use one (or two) piece(s) of acrylic, metal or wood laid across the top (and/or bottom) of the brace to bridge the gap. Clamp, drill through both/all pieces and bolt them together;
3) Same as (2) above but use epoxy instead of bolts.
Any of these options will work 'swimmingly' and the aesthetics will depend on your skill and allegiance to same. Don't fret - when you're done it will be stronger than it was new and can be managed in a way that isn't even noticeable, particularly if you use option 2 or 3 and make the supports flush all the way to the front and back of the tank.
PS - Occurs to me that if they tried to lift the tank using the brace then the attachment points for the original brace may be compromised if it's a separate piece from the rest of the frame. I that is the case then on reflection I would get a piece of acrylic the same width as the original brace and simply cement it in right under and to the old one so that it reinforces the original attachments.
the attached image is euro bracing
