You are adding an Electronic Fuel Injection system ?
I guess you also want to know on which cycle the engine is on, maybe from the camshaft.
Is the hall effect sensor positioned in such a way that it triggers at the appropriate point ?
You should be able to "reverse engineer" an Arduino hall sensor based tachometer design to work out how to trigger an output. Look for one in which the hall sensor is used to trigger an interrupt. In the interrupt service routine, you can do non blocking activities like say switching led on as a test.