Right about now it's time to to rock with the biggity Buck Bumble!
At the main menu, hold Z and press (on the D-Pad) Right, Down, Down, Right. Release Z, then press (also on the D-Pad) Right, Up, Down, Left, Left, Up, Right, Right. A sound effect will confirm the input, and a level select will be enabled when starting a game.
Unlimited Ammo, All Weapons & Refill Health Cheat
A cheat that the internet got wrong for many years can finally be input for this game correctly. The cheat needs to be input at the main menu, like the above cheat. Press the following buttons at the main menu: Up (on the D-Pad), Z+Left (on the D-Pad), Down (on the D-Pad), Right (on the D-Pad). A sound effect will confirm the input. Once in the game, press A+B+R to activate it as many times as you want.
The Japanese version was made harder by having enemies spawn faster and giving them better AI in certain levels.