The commercial pilot stated that there were no anomalies during the preflight inspection of the airplane or before departure. At the time of departure, the left- and right-wing fuel tanks were ½ full, and the fuselage (rear) auxiliary tank was empty. About 18 minutes after departure, both communication radios began to flicker, followed by a loss of electrical power to the avionics; none of the circuit breakers were tripped. While troubleshooting, the pilot reset the alternator circuit breaker, but it did not restore electrical power. He then diverted the flight to an alternate airport, and, during final approach, the engine lost total power. The pilot located a field encircled by trees to perform a forced landing. During the approach to the field, he attempted twice to start the engine but was unsuccessful. The pilot said that he was unable to reach the field, and the airplane impacted trees with the left wing during the approach.Postaccident examination revealed the airplane's fuel tanks were intact, and the fuel caps were in place and secure. The fuel tank filler necks and bottom of the fuel caps were brown in color consistent with corrosion. The cockpit fuel tank selector was positioned to OFF, and the cockpit auxiliary fuel tank selector was positioned to RIGHT WING. The right-wing tank contained about 9 gallons of fuel; the right-wing auxiliary tank was empty; the left-wing tank contained about 1 gallon of fuel, which was an unusable amount; the left-wing auxiliary tank contained about 8 gallons of fuel; and the rear auxiliary tank was empty. The pilot did not report the positions of the fuel selectors that he had used during the flight.The fuel lines between the fuel pump and the fuel metering unit and the fuel line between the fuel metering unit and the fuel manifold valve contained no fuel. The fuel manifold cap was removed, and no fuel was present in the manifold housing. The manifold screen did not contain debris, the diaphragm was intact, and the plunger displayed no anomalies.Testing of the fuel tank sending units revealed inaccuracies in their resistive values that would have resulted in fuel gauge indication inaccuracies. The left-wing auxiliary fuel tank sending unit had no change in resistive value with float position, and the right-wing auxiliary fuel tank sending unit had Page 2 of 8 CEN18LA010minimal difference in resistive value between mid- and upper-level float positions. The left- and right-wing fuel tank sending units' resistive values at the upper limit of float travel were similar. The resistive specifications for the airplane were unknown. There was no record that the fuel senders had been replaced since the manufacture of the airplane.Both magnetos were unable to produce a spark when rotated by hand and during bench testing due to long-term wear of the magneto pick-up brushes. A degraded performance of the magneto should have been evident to the pilot during a preflight run-up check by a greater-than-normal reduction in engine speed when operating on one magneto. There was no record of a magneto inspection in accordance with the magneto manufacturer's maintenance instructions for continued airworthiness. Although it is possible that the pilot starved the engine of fuel due to improper fuel management or that the magnetos had degraded performance, based on the available information, the investigation could not determine the exact reason for the loss of engine power.