Note: you must have a controller connected to port A and B.
> Toggle AI firing on/off:
Using controller #2, hold down L+R and press A.

> More health and ammo:
Using controller #2, hold down L+R and press X. 

> Toggle game mode on/off:
Using controller #2, hold down L+R and press B.  When game mode is off, you become invulnerable and receive unlimited rocket ammunition.
(courtesy: GameFan)