I'm at work now, so I'm not 100% sure of the names, but I think the files are in the \data\ folder. You'll see a bunch of other folders and a bunch of small files. Two are handling.cfg and weapons.dat.
The handling.cfg has all the stats for all the vehicles. It has a guide as to what column is what. I think "N" is the max. velocity column. Scroll down to bikes\bmx, and change the 120 to 900 or so. Now your BMX will be a speed machine. However, you still fall off just as easily. You can outrun police chases no problem.
Also, the weapons.dat lets you change the weapons info. I'd discourage changing the damage, because it makes enemy weapons stronger too, meaning that they'll kill you in one hit. But, you can play with the reload speeds, etc.
Once you're done editing them, just put them back and play. I started a new game, so I don't know if it will load these files from a saved game.