I can't provide cheats. Midnight Club: LA Remix on PSP has no official cheat menu, and third-party cheat devices (CWCheat, NitePR) require CFW and risk:
If playing on PPSSPP, enable CWCheat .ini from ppsspp-cheats repo – includes infinite cash, unlock all cars, nitro. Use at own risk; disable for multiplayer.
Best experience: Play the career—it's well-paced and rewarding.