Type these cheats where it says "Name" on the single race screen:

!Scooter! = You ride a scooter, wearing a suit
Ghostriders = You become a ghost
BeamTeam = New bikes
RedJocks = You ride in your underwear
!Junior! = You get Kenny Roberts Jr. 1999 Bike
!Maaate! = Get Doohan's NSR '94 #4