Does an OBD2 stock ecu look for timing itself or is it only based off the responses from the crank and cam sensors? Reason; Could an F20b run off a 97 GSR ecu, using GSR harness and so on if I bypass the crank sensor? Would it pass OBD2 emissions, plug-in only, maybe idle for 15 minutes then swap the ecus back to a tuned OBD1 unit?

 

The ECU needs Crankshaft Postion, TDC, and Cylinder Postition sensors in order to run the engine. If your F20b has four wires coming from under the lower timing cover, then you will need to have these hooked up, or use a distributor that has all three sensors internally.

If by chance you are talking about the CKF (crankshaft fluctuation sensor) then you can bypass this if needed and it will still pass obd2 emissions scan