The airplane sustained substantial damage on impact with trees and terrain during a forced landing to a field following an in-flight loss of engine power. Skydivers had been dropped prior to the loss of engine power and the pilot reported no injuries. The pilot stated, “I climbed to 11000 [feet.] Was not getting usual climb rate. Before decent found I could not close cowl flaps. Decended to 6000 feet. Noticed eratic raise on manifold gage.