I believe this is one of those double edge sword type of things where: (don't quote me on these)
1) You purchased the game and therefore you can legally make a "backup" copy for only your own use.
2) The PS2 disk and console both contain a form of copyright protection
3) In order to make a valid copy of the disk you need specific software
4) even if you were able to make a copy of the disk, the console copyright protection software would not allow you to play the "backup" copy of the game
5) in order to play a copy of the original disk, your console would need to be modified
6) I don't think it is specifically illegal to modify your console, however, I think there is a gray area where it may be illegal to modify your console in order to circumvent copyright protection.
As I said do not quote me on this. I think it's always been a gray area. I believe with the newer consoles, like the Xbox 360 Microsoft was banning consoles that were tampered with from the LIVE network.
My suggestion would be to just go to Game Stop and buy a used copy or eBay has them for $10 or less.