The son of a prominent lawyer was found guilty Friday of selling forged documents claiming President Kennedy paid hush money to keep secret an affair with Marilyn Monroe. Lawrence X. Cusack III, 48, of Fairfield, Conn., who made a fortune selling hundreds of Kennedy-linked documents he claimed came from his late father, was convicted on 13 mail and wire fraud charges. At his sentencing in July, Cusack faces up to five years in prison on each of the 13 fraud counts, and a maximum $250,000 fine on each count. According to the papers, Kennedy had set up a trust for Monroe’s mother to buy her daughter’s silence about the alleged affair between the president and the actress. Cusack produced letters that he said were written between his late father, who helped handle the Monroe estate, and the slain president. The documents appeared to prove rumors about Kennedy and Monroe.