On the menu screen hold the joystick in port 0 to the left and the
joystick in port 1 to the right, for about five second. A cheat
menu will appear, enabling you to set player 1 and 2 to be the evil
max, to toggle the cpu as player 2 or to alter your credits and
energy.
If the characters have shadows beneath them whilst playing, then
you've got a copy of the enhanced version. Hold the joystick in
port 1 down, until the cheat options appear.