Just go to GameStop/EB Games or even Best Buy (Possibly Future Shop) and ask them to do there magic, They fix it somehow and you can play it fine.
If not , Go to BlockBuster or where ever you go to rent games , rent MW2 on whichever Console you have , play with it for the time you rented it and when its time to return it switch out the CD's. (That's ONLY if the CD doesn't have a written number , if it does try to buy the same pen and write it the same way).