When it comes to brakes....EVERYTHING should be fixed first. You say the pedal is frozen? Disconnect it from the pushrod, is it still frozen? If so, its simple then....pull it apart, clean up the corrosion or whatever and put it back together.
IF, though, the disconnected pedal moves freely, then check the movement of the pushrod into the cylinder. I'd believe the master cylinder is far more likely to be frozen up than would be the pedal. If so, replace it.
Has this thing been sitting for a long time? If so, and corrosion has frozen either the pedal OR the cylinder, then the whole system needs serious inspection before you decide to go out and crash it through a guardrail.
Brakes are no place to take chances.