This will only work if you can store your original game on the HDD of the Xbox.
FTP onto your Xbox and find where you have stored your game.
Now in the PGR2 folder there will be a folder call "car dynamics".
Now make a backup of this folder (just in case).
To get any car on any track just do the following.
Find a car that you can use on your chosen race.
Find a car that you want to use.
Now rename the file of the car you "can" use with the one you "want" to use.
If you like you can open the file in a text editor and actualy change the handeling,speed, spec of the car.

The car will show as the original car but will have the dynamics of your chosen car.

Enjoy.