I have big issues when people do this on purpose, but in your case is needed. That brace allows the load from the water trying to push your glass box apart to be transfered evenly through the system. Removing this brace as most do lessens the factor of safety built into the design. Sure, it may not break under normal conditions, but may when that two year old is slapping the glass one day. You are wise to replace the brace, I would consider and justify an tank upgrade because of this.
A couple of suggestions:
The brace and the fastening mechanism you use needs to transfer that same load, a combination of cement and mechanical fasteners would be you best bet.
When choosing your fastener, more/smaller is better than a few larger. Which is tied to the last suggestion I have. As well, when the bolt gets bigger, one tends to apply more torque when tightening and this may lead to distortion and cracking.
Predrill your acrylic and use only Stainless nuts and bolts making sure you use flat washers to distribute the load. When drilling the holes, the edges of the adjoining holes need to be at least 2 diameters of your bolt diameter from each other as well as the edge of the material to avoid cracks from propogating.
When joining the two, overlap the pieces as much as you can, even leaving the old one on and "sistering" the two together for the best fix all together. The combination of the two materials and more/greater spaced fasteners will be the strongest repair.
If it is a standard AGA brace, as a place to start I would think 6- #12's on each end may work if they fit the above conditions.
If it has been totaly removed, be sure to sand and round the raw edges as crack could propagate.
Hope it helps.....
