I tinkered the files to make the Cop Car stealworthy. At first nothing wrong was happening, but as soon as I unlocked it now almost everytime I start a race the game freezes and CTDs.

How can I correct this?

Only thing I can think of is reinstall.

Tinkered? IWANTTOFIDDLE in C1 can corrupt text files. So to fix replace text files with fresh.

Don't forget to backup your savegames, since AFAIK (in Steam for sure) installer replace savegame files.

To allow game to read files in text form you need proper PROG.ACT.
Then you can take selected text file/s from mod like Meld Pack (PROG.ACT too).
APC.TXT from Meld Pack probably require some more files to be added (like grid icon).

Or you can install
(after 95RPP http://qtz.toshiba-3.com/uMaxPack95RPP.zip)
http://qtz.toshiba-3.com/Unofficial_Carmageddon_Patch.zip.
And for SP: http://qtz.toshiba-3.com/splat_pack_patch_alpha.zip

i kno im a year old but im curious as to why you had tinker with any files to steal the cop car?

its very easy to unlock just by playing the game regulerly