That thing is a pretty good sounding sub, while it is working. It's a royal PITA to work on. Chances are, if you could find the replacement cone, it would likely still not work quite right when done.
I've not got first hand experience with it, but a friend of mine has. He was able to salvage a new driver's cone where the basket was damaged in shipping, causing the magnet alignment to shift slightly, enough to be a problem but not enough to damage the coil so insurance covered it.
He blew one driver later on, having kept the damaged cone from the original one in case as would be the case, he blew one. but he said it was not easy and if he'd had to pay, would never do it again.
Too many really good subs on the market that exceed it's performance anymore. So,unless you're are trying to match another in a set, for the price of those new, you could get a nice Si SQL 12 or a pair of NVX VC series 12" subs.