On March 2, 1994, approximately 1435 Pacific standard time, a Grumman G-164, N6612Q, nosed over and sustained substantial damage after a loss of power while in a descent, eight miles southwest of Walla Walla, Washington. The commercial pilot, who was uninjured, was conducting aerial application operations under 14CFR137. There was no fire, and no ELT was installed in the aircraft. The pilot stated that while on descent, the aircraft lost power, and he attempted a forced landing, but the aircraft nosed over during rollout from the landing, due to soft ground. He stated that he was on his sixth load of the day, and the second flying day of the season. When the engine was inspected it was determined that an engine cylinder had split. The pilot/operator believed that a magneto micarta gear had failed, resulting in the engine cross firing, blowing the door off the air cleaner when it backfired and splitting the cylinder.