Different sizes of new cones are cheap. Cut away the old cone leaving about an inch of old cone still attached around circumference of the voice coil, glue in the new one. The ones sold below have foam surrounds which might cause a problem with "bottoming" of the voice coil on loud bass so it might be necessary to devise a way to stiffen the surround a bit.
http://www.electronix.com/advanced_s...r+cone&x=0&y=0