To make it a reliable sensor, all causes of potential failure from the environment must be considered, like automotive. Waterproofing, heat , rust, rodents eating PVC wire insulation, insects clogging exposed sensors.
One novel approach is an electret microphone near the engine and detect the exhaust noise envelope to drive a transistor switch with pullup and filtering ambient low level noises. This could be a quasi-peak or R-C-diode series detection method that pumps up a larger capacitor to drive a FET switch with open drain & pullup R to the GSM transponder logic input. Time constants of >> 1 second could reject ambient noise bursts. This could make a solution with common parts with waterproofing. 5V from GSM can power the mic. with low current. It can also all be done with CMOS. Inverters with linear gain & hysteresis.