Select "Options" from the main menu,then enter the "Player 1 Setup" option and select "Name." Enter any of the following names to unlock the specific hat.

BUBBA-Unlock all riders
ROADKILL-Unlock all tracks
GENERALLEE-Unlock allvehicles
DOUBLEBARREL-Unlock all challenges
REDROOSTER-Unlock all championships
GINGHAM-Unlock-Max.stats
FIDDLERSELBOW-Max.skill level