Pizza Hut, a division of Pepsico, completed its previously announced purchase of Straw Hat Restaurants and said that many Straw Hat franchisees will be given the option to join the Pizza Hut franchise system . . . Alcoa's fourth-quarter 1986 results will include a net gain of $138 million from the combined effects of a number of unusual items . . . Ciba-Geigy AG, Switzerland's biggest chemical and pharmaceuticals company, said it will begin production in China in 1989 under a joint venture with a Chinese manufacturer . . . Norton Co. said it completed its acquisition of Carborundum Abrasives from Peck-Lynn Group of Chicago.