Operating frequency can be at 350 or 440 MHz, range is limited to ~100m. Typical protocols are similar to IR remote controls for TV and VCR but coded pulse train is transmitted with a code specific for a given car.These frequencies are licence-free under the condition of the range limited to 100 m.
Technically one can use an amplifier or a directional antenna at the transmitter but then license-free conditions are broken. If spectrum inspectors catch you, fines are tough and suspicion of trying to steal cars may bring you problems.
Your project is not a reasonable one.