Unlock All Bonuses
At the Cheats screen, enter CLAPPEDOUT.
Open Up Pro Physics
Enter SWAPPIN in the character completion field.
50cc Class
Enter SQUIRRELDOG in the character completion field.
Even more cheats
Enter the following cheats in the Character Completion field at the Cheats screen:
Password - Effect
BIGDOGS - 500cc bikes
BUSTBIG - Freestyle tracks
CLAPPEDOUT - All bonuses unlocked
ECONATION - National tracks
MINIGAMES - Unlock all machines
OBTGOFAST - Expert A.I.
SQUIRRELDOG - 50cc Bikes
STUPERCROSS - Supercross tracks
SWAPPIN - Pro Physics
WRECKINGBALL - A.I. Bowling