Choose the Cheat option at the Setup menu and enter the following:

All Boxers:

Start, A, Y, X, X, Z, Z, X, Y, A, Start

Invincibility:

Start, A, A, Y, Y, Z, Z, X, X, Start

Intercontinental Tourney:

Start, A, A, A, Y, Y, Y, X, X, X, Start

Conversation Mode:

Start, Z, A, Y, X, Z, Z, Z, Start

Boxers Life Mode:

Start, A, X, Y, Z, A, X, Y, Z, Start

Scrap Yard Arena:

Start, Y, Z, Y, Z, A, A, Start

New Outfits:

Start, A, Z, Y, X, Start

Turbo Mode:

Start, Z, Z, Z, Z, Z, Z, Z, Z, Z, Z, Start.

Rapid Power Up Mode:

Start, A, Y, A, Y, A, Y, X, X, X, Start