I have had my Xbox 360 for around 2½ years now, and it is starting to break a lot of my games and not work. I have had MW3 for 2 months, and I have replaced it 3 times already. It also broke my BF3 the first day I got it :( I can't afford a new Xbox now, and I was just wondering if I can at least replace the parts.
If you are asking if you can use a hard drive meant for the new style 360 (the Slim model) with an old style 360, the answer is you can't. The original hard drives attach to the outside of the console, while the new ones are smaller (physically) and slide into the console itself.
As for replacing other parts - I would recommend against doing any maintenance yourself. You'll void your warranty (if it is still active), and in general the console isn't meant to have user replaceable parts.
Re-reading your question, do you mean that the optical drive is scratching your discs? You might try installing the game to the hard drive. It will still need to read everything on the disc to copy it, but at least once that is successful it won't have to read anything from the disc (aside from the initial check when starting the game).
The optical drive in the Xbox 360 has special firmware that "locks" a particular drive to a particular Xbox 360. Even if you could get the drive out of the unit (One repair manual has a 29 step process that recommends you have specialized tools for the process!), you couldn't replace it with another.
If your Xbox 360 is damaging discs, your best bet is to contact Microsoft and find out how much the repairs might cost you, or buy another Xbox 360.