Get the MM2 Ultimate Tweaker program from
here, extract it in a folder, run it, open any of your cars selecting them from MM2 game folder with the left side lists and move to the left the indicator bar of the break power, to reach the -10.00 value, to get the NOS effect.
You can do this to any car that the program is able to load; it doesn't work with the default in-game cars.