At the main screen put in Up, C-Left, C-Left, C-Right, Left button, Right button, C-Down, and Z.
Stages
Stage 2:
At the main menu, press R, C-Right, Z, R, L, C-Up, C-Left, C-Up.
Stage 3:
At the main menu, press R, C-Right, C-Right, C-Right, R, C-Left, C-Down, Z.
Stage 4:
At the main menu, press R, C-Right, C-Down, C-Left, C-Right, C-Right, Z, L.
Stage 5:
At the main menu, press Z, C-Right, C-Down, C-Left, C-Right, C-Right, Z, L.
Extra Bikes
Use Big Game Mode to finish all the races to get 2 new bikes capable of 300mph+
The Mystery Level
Get at least 52 batteries and assault a cop 19 times in the insanity level,stage 2. You'll hear a chime at main menu press c-up c-down(2x) z z R L L c-down c-right FAST. You won't hear a chime this time go to level 5 get 52 batteries in stage 5 and you'll get the code. Its tricky but you'll get it after the first 10 times, don't lose patience.