On the screen with New Game/Continue, enter these cheats quickly:

• God Mode: UP, DOWN, LEFT, RIGHT, LEFT, LEFT, RIGHT, RIGHT, L, L, R, R, WHITE, BLACK.
• Infinite Ammo: UP, RIGHT, DOWN, LEFT, UP, RIGHT, DOWN, LEFT, L, R
• Fat Head: RIGHT, UP, LEFT, DOWN, RIGHT, UP, LEFT, DOWN, RIGHT, L, R, L, R
• Unlock All Insta-Action Stages: UP, DOWN, LEFT, RIGHT, L, R, L, R

To unlock the X-wing and Tie Fighter in Instant Action Mode, beat all 15 missions and all 22 challenge missions.