You have a few options in this area. I have two tanks that have had broken center braces. I ended up asking for help on a few forums and got advice that allowed me to fix both.
One way to solve this problem is to replace the center brace with a glass brace, typically referred to as euro-bracing. With your tank being a bow front, cutting the one end of the center brace on a curve to match the front of the tank could prove difficult.
Depending on how the brace is broken you could follow the basic instructions in this thread:
http://www.monsterfishkeepers.com/forums/showthread.php?t=66111&highlight=replacing+brace
This is basically what I ended up doing on my 45 tall, which you can see in the tanks and equipment gallery. I did not replace the center brace with clear acrylic like LondonDragon did; I epoxied a piece of opaque acrylic over the top of the center brace. This worked well on this tank due to the break being right in the center of the brace.
On my 55 gallon tank the brace was broken off at the front edge of the frame, and there was nothing to attach a new brace to, and I was not confident in my ability to pull off a successful euro brace. So I ended up ordering a new frame through my local fish store. It was less than $20, and it was really easy to put on. Getting the old frame off was initially difficult until I discovered a cheap bread knife is the best tool to cut through old silicone and plastic frames.
I hope this helps you keep this tank in service
Or you could use this as an excuse to get yourself a bigger tank.