1997 Ford Contour P1131 and P0171 Lean Mix Error: Causes, Diagnostics & MAF Sensor Considerations

I own a 1997 Ford Contour with a 4-cylinder engine, currently at 107,000 miles. I'm experiencing several driving issues: the vehicle sometimes stalls after starting, has rough idling (and near-stalling), exhibits slight vibration at steady speeds, and accelerates sluggishly. On warm days or after long drives, it frequently stumbles or stalls at traffic lights. The check engine light is consistently on. I've already replaced the spark plugs, fuel filter, and air filter. After these changes, symptoms improved temporarily—idling became smoother and no error codes appeared. However, within about 45 minutes, the issues returned to their original state. The OBD2 scanner shows persistent codes P1131 (O2 sensor circuit range/performance problem) and P0171 (System too lean). A second diagnostic tool confirmed these same codes, but no misfires were detected. A mechanic checked for vacuum leaks and found none. Spark plug wire connections were also tested and deemed intact. Given that symptoms worsen in warm weather, the mechanic suspects either a faulty crankshaft or camshaft position sensor. However, I've noticed that the Mass Airflow (MAF) sensor is often cited in relation to P0171 issues on this model—though it wasn't discussed during my inspection. Before replacing expensive components like sensors or ECU-related parts, what are the most likely root causes? In addition to the primary issue, there's a rust hole in the muffler and a persistent 'tick, tick, tick' noise coming from the left side of the engine—this has been present since I purchased the car in November 2007 and hasn't worsened or increased with speed. Besides MAF sensor cleaning, what other diagnostics should I prioritize before moving to costly repairs?