dknguyen
I think compensating for the range of brightness of the sun versus your reflected
intensity will be quite difficult.
You might also consider a typical IR detector approach with carrier frequency of 38K or higher. Even with an RPM of 10K (2-bladed prop), you should be able to differentiate modulated IR from background IR.
The easiest approach for an on-board sensor given your concerns might be a Hall detector with a magnet on the crankshaft. If it is an electric ignition model, that probably already exists. If fully electric, then go with the back emf. If compression ignition, then add a magnet.
John