The Federal National Mortgage Assn. voted to double its common stock dividend to 8 cents for the third quarter of 1986. The nation's largest single supplier of conventional home mortgage funds, Fannie Mae has been paying 4 cents per share since the third quarter of 1981 . . . Mayflower Corp., the Carmel, Ind., school bus and trucking company and home-products distributor, said it had accepted a sweetened $252-million buyout proposal from a management-led investor group . . . Conversion Industries Inc. of Pasadena, a waste-to-energy power plant developer, said it had agreed to buy 51% of Perennial Energy Inc. of West Plains, Mo., which sells small engine-generator sets for use with a variety of fuels . . . Novan Energy Inc. of Boulder, Colo., said it has agreed to purchase Industrial Water Treatment Inc. of Los Angeles . . . Wurlitzer Co., the electric organ maker of Dekalb, Ill., said it has agreed to sell a majority interest in the company for $1.925 million to a group of investors led by Sid D. Weiss and and Leonard E. Friedman of Houston.