On July 8, 2017, about 1050 central daylight time, a Beech VA airplane, NV, sustained substantial damage during a forced landing after a loss of engine power near Walters, Oklahoma. The airline transport pilot and one passenger sustained minor injuries. Visual meteorological conditions prevailed and no flight plan was filed. The airplane was registered to Gas Corporation of America of Wichita Falls, Texas, and the flight was being operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. The flight originated at 1040 from Kickapoo Downtown Airport (CWC), Wichita, Falls, Texas, and was en route to Lawton, Oklahoma. The pilot reported that about 10 minutes after departing CWC, the oil pressure went to zero and the engine failed. He executed a forced landing to a soft farm field, about one mile south of the Walters Airport (OK12). The pilot and passenger exited the airplane with minor injuries. The airplane sustained substantial damage to the firewall and lower fuselage. A review of the airplane maintenance records showed that the vacuum pump was replaced on July 7, 2017, the day before the accident. The engine was visually examined and disassembled. During the disassembly, it was discovered that a improper gasket (P/N-21051) was installed on the vacuum pump. The gasket was normally used for the installation of a carburetor. The installed gasket did not completely cover the high-pressure oil orifice on the vacuum pump mounting fixture. The proper gasket was (PN-MS9134-01) for the vacuum pump. Three connecting rods showed thermal and impact damage, and three rod journals were observed to be dry and showed thermal discoloration.