If the crack is too deep into the surface, then it could be the internal metal(the actual part that is read) and not the outer layer of...I am thinking it is a plastic material, but if it is deep enough to where it damages the actual ridges that are read by the laser then I am not sure that there would be a way to fix it. Otherwise it should be able to be fix by filling in that crack using a special repair kit, or by sending it into a place to get repaired such as like Jrmagic suggested Game Stop could have a look at it I would imagine.
Take it to a video game store like Hollywood video? They refurbish games in my town and it's very cheap as well. Ask around for places that refurbish games. Also how deeps the crack? Is it really a crack or a big scratch? This information can significantly make a difference.
do not totally dismiss this and please try it because it worked for me.
1. if you fill the crack in, the xbox decsides not to acknolodge there is one but you cant just fill it with anything
2. whats the answer? toothpaste! yeah thats right.
3. get a tiny amount of toothpaste on a crack and rub it over the crack, do not force it just let it go over.
4. depending on the content which is scratched you may not be able to play that bit for example if the game contiues to freeze during one certain part for example where you walk avoid it.
5. if your lucky it would skip the scratch and cause a little second or lag like it did to mine
NOTE - I do not claim that this 100% works but it did for me! try it if all else fails
