Sorry to tell ya, but that 1/4 is wasted! Sure, you can grind all the body filler off with a "mud Hog", but me personally would never have bought a late model car that hacked together. Who knows what other half assed things were done to it that are unseen. With that said, If you bought it to practice bodywork on and got the car for $500-$800 than you did OK. Your options are getting a 1/4 from a salvage yard. (You'll have to remove the outer from the inner using a special spot weld bit). Or get an aftermarket one for about $125-$250. (Depending on your contacts). Most important is to make sure the trunk pan isnt bent, and the gap from the deck lid to the 1/4 is good.
Good Luck whichever way you go.