I built an auto starter for a generator.
Used a hall effect sensor looking at the starter teeth of the flywheel. Fed this into TIMER 1 external clock input, and used it to count the number of pulses (teeth) over a time period. I then did some tests and measured the fastest it could crank when warm, then set a threshold about 20% higher. Never had a problem with it.

For the record, I built in lots of safety features, which included low voltage cutout, under speed cranking, low oil pressure, over temperature, with visual and audible warnings before starting and an emergency stop switch.

I was able to have a remote control panel in the house where I could display and configure the parameters or manually start the engine and switch over the electric contactors. Never used it in manual, auto was fine.