The Toyota Motor Corporation said yesterday that it would invest $250 million to nearly double the size of its engine plant in Huntsville, Ala., creating 300 jobs. The expansion would increase the number of employees there to 800 and raise annual production by 150,000 engines, to 400,000. The plant would supply engines to Toyota's Indiana truck plant for the Sequoia S.U.V. and the Tundra pickup truck. It would also supply a new plant being constructed in San Antonio, where the next generation of Tundra pickups will be built, starting in 2006.