Start At Piston Honda
At the title screen choose password selection and type in 032 730 8442. You will start fresh and clean, but fighting Piston Honda, so you skip through the first two or three other boxers.
Start At Super Macho Man
At the title screen choose password. In the password screen enter 267 913 7638. You will start fresh and clean at Super Macho Man. This let you skip through the first 5 or 6 fighters, but beware, Super Macho Man is tough to beat.
Start At The Second Title Circuit
At the title screen, choose password. Enter 005 737 5423. If you entered it correctly, you will start at the second major title circuit, skipping the first one.
Tyson stage
If you want to challenge the master, (Tyson), enter this password at the title screen under the continue option: 007 373 5963.