How did you manage to stiffen brass with heat? Usually works the other way. Also, what do you mean by "heated it until it broke?" Brass and other nonferrous metals are heated and cooled to soften them. They don't break because of heating, unless you melt them, in my experience, but because they work harden too much.
I think getting a new guard is your best bet, but, if you want to try to repair it, they do make some solder that doesn't show badly on brass. Don't think brazing is likely to work, but I don't know much about brazing.