Also this is what PSI had what to say, pretty much the same thing.
We can help you rebuild this subwoofer into a 12", but we cannot guarantee it to handle the huge amount of power that you plan on using. Your 18" subwoofer is going to be more capable of handling high power, even with the same type of parts installed. It's just bigger and takes more to move, and more to destroy.
We can set you up with a recone kit that's built with the largest voice coil that we have - 107 XL 1.4, 5 10" linear roll spiders, triple sewn heavy duty interwoven leadwires, 8gauge XS-Flex OFC direct connect wiring, HD pulp cone with mega roll foam surround, polypropylene dome dustcap, a 12" TI style frame, a gasket, and the glue and shims needed for assembly all at a total base price of $370 shipped.