Try copying it to your HDD. If it already is or that does not work, rent a copy from a rental video store and delete the old BOPS2 off your HDD then copy the rental disc on. You should still be able to use your older disc to play it off your HDD just make sure you don't clear it off by accident.
I'm sure gamestop would help you out if you brought your Xbox into them. They might even try cleaning up your disc for you with more advanced tools. Depends on how nice the guys working are.