Couple of things to think about.
1. What is the cog range of your rear cassette? 11x21 wouldn't work to well.
2. What chain rings does the crank have? Most cross bike use 48x39 or something close to that. Or they run single rings up front 42ish...
3. Pay attention to the rear spacing on the frame. I would guess that your hub is 130mm and some of the newer frames are 135mm... So check both before you by.
Here are a few frames that list rear spacing that would work with both 130 and 135 hubs...
http://www.surlybikes.com/crosscheck.html
http://www.somafab.com/frames.html
I browsed the web site for a cross bike specs at cannondale, surly, and others to see what they were doing to give me ideas.
Hope that helps...
Also, you might check to see if the race allows mountain bikes. You might try that before spending cash on a new bike. But I understand the "need" to build, it can be a sickness.
Good luck,
Kevin