On July 1, 2004, about 1430 eastern daylight time, an amateur built Zenair CH 701, N98EH, was substantially damaged during a precautionary landing, following a partial loss of engine power after takeoff from Mansfield Lahm Regional Airport (MFD), Mansfield, Ohio. The certificated private pilot was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local personal flight conducted under 14 CFR Part 91. The pilot reported that he tookoff from runway 23 at MFD. Over the departure end of the runway, the pilot noted that the engine power was 100 rpm lower than expected, and the oil pressure decreased to 0. The pilot initiated a left turn for a precautionary landing on runway 5, a 5,795-foot-long, 150-foot wide, asphalt runway. The engine continued to lose power during the approach to the runway, but did not experience a total loss of power. The airplane was not aligned with the centerline as it touched down on runway 5, and traveled off the left side. The airplane subsequently struck runway lights and came to rest inverted in a grass area off the left side of the runway. A Federal Aviation Administration inspector examined the airplane. The examination revealed that the pilot had recently installed a homemade oil cooler adapter on the Continental C-75 engine.