You will need-

Forza Motorsport (1 copy)
Modded Xbox
Xbox 360
Crossover Ethernet Cable

Preperation-

1) Make sure you have Forza Motorsport saved to the HDD of the original Xbox. Also check it can load the save with your custom cars.

2) Connect the Xbox 360 to the Xbox using the Ethernet Crossover Cable

3) Finally load up Forza Motosport using the games disk on your Xbox 360.

Method-

1) On your original Xbox load up your gamesave and go to Multiplayer > System Link > Create Match. Make the Game type buy and sell cars.

2) On your Xbox 360, load you profile then go to Multiplayer > System Link and join the created gametype.

3) Finally buy all your cars across on you Xbox 360 profile.

But if you have LIVE on your 360 it won't Work

why won't it work?

you have a update from xbox LIVE and you can't play
whit them that don't have it

you need to ensure that both your copies of forza are patched to the same level for this to work