Rite, I feel the need to help you as it seems that no-one else will (lol)
Here are the points I would like to make:
1) To "Hack" Mat Hoffman BMX, it will take a lot more than changing a couple of Hex Values (like modding HALO). You would probably need access to the source code to have a clue where to start.
2) You would need to learn assembly to look through a disassembled version of this game, which, by the time that you have learn't this, you could have saved up enough of your pocket money/wages to buy the game (think it only cots about £12(gbp))
3) THIS IS ILLIAGAL. I understand why many people would like to hack some of the software that they encounter (eg, windows, oracle) but that is because these companies are money grabbing theives, who pray on people who know no better. But companies like ACTIVISION, and all the people that helped make the game (Matt Hoffman, Corry ...) did a bloody good job, so maybe you should just go out and buy the game.
Hope this helps.