Willis Lease Finance Corp. said it will buy 12 Boeing 747-100 aircraft and additional spare engines from UAL Corp.'s United Airlines for an undisclosed amount. Willis Lease plans to lease the airframes and the Pratt & Whitney JT9D-7A engines in the aircraft. Airframes and engines not suitable for leasing will be disassembled and sold for parts, including modules, landing gear and auxiliary power units. The Sausalito-based company leases spare aircraft, engines and parts to commercial airlines and manufacturers. It also refurbishes planes and parts. At the end of last year, Willis Lease had 44 engines, eight aircraft-parts packages, and three commuter aircraft with a value of $148.4 million leased to 33 customers in 20 countries. Willis Lease shares rose 81 cents to close at $21.94 on Nasdaq.