The following are known Action Replay Codes for Metal Walker on Nintendo Game Boy / Color (GB / GBC). These codes were also known as GameShark Codes and in most cases will work on both devices.

Device Compatibility: Action Replay & GameShark

Infinite HP
91ff19ca
91ff21ca

Infinite Scrap
910fe3c1
9127e4c1

Max Level
916310c8

Max Defense
91fff4c7

Max Attack
91fff2c7

Max Move
91fff6c7

Max Weight
91fff8c7