To unlock everything in the game, insert the cheat YOUGOTIT into the in-game cheats menu. If inserted properly, everything will be unlocked.
To unlock all of the handlebars available in the game automatically, insert NOHANDS into the in-game cheats menu.
To unlock all FOX-related riding gear, head to the in-game cheats menu and type in the password CRAZYLIKEA.
To unlock all in-game graphics for your vehicles, insert "STICKERS" (minus the quotes) at the in-game cheats menu.