Here are some cheats i know of that mmight help you all.
Play as Mecha Godzilla 2
Successfully complete adventure mode with all monsters, then complete adventure mode again with Mecha King Ghidorah.
Play as Mecha Godzilla 3
Successfully complete adventure mode as Orga on the hard difficulty setting.
Vortaak level
Successfully complete adventure mode with Mecha Godzilla 3 on the medium difficulty setting.
Gallery photos
Play through the game on hard difficulty setting. While fighting, various buildings that you destroy will have special Atari power-ups inside. Collect these to unlock a gallery picture.
Cheat mode
At the main menu, highlight "Versus" mode, then hold L + B + R (in that order), then release B, R, L (in that order) to display the cheat entry screen. Enter one of the following codes to activate the corresponding cheat function:
All cities and all monsters
Enter 863768 as a code. If you entered the code correctly, you will hear a sound.
Monster Island level
Enter 745749 as a code. If you entered the code correctly, you will hear a sound.
Mothership level
Enter 972094 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Mecha Godzilla.
Boxing ring level
Enter 440499 as a code. If you entered the code correctly, you will hear a sound.
All cities
Enter 107504 as a code. If you entered the code correctly, you will hear a sound.
All gallery items
Enter 962129 as a code. If you entered the code correctly, you will hear a sound.
All monsters except Orga
Enter 753079 as a code. If you entered the code correctly, you will hear a sound.
All monsters
Enter 209697 as a code. If you entered the code correctly, you will hear a sound.
Play as Godzilla 2000
Enter 637522 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Godzilla 90s version.
Play as Gigan
Enter 822777 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Aguirus.
Play as King Ghidorah
Enter 939376 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Megalon.
Play as Rodan
Enter 724284 as a code. If you entered the code correctly, you will hear a sound. Alternately, successfully complete adventure mode with Gigan.
Play as Destoroyah
Enter 352117 as a...