Stephen Frederick Owen Jr., 69, a lawyer in private practice, died of a heart attack Feb. 27.      Mr. Owen was an attorney for the Securities and Exchange Commission until 1984, when he moved to the law firm of Loomis, Owen, Fellman, & Howe and became senior partner. He practiced there for 26 years.      Since 1990, Mr. Owen had been engaged in the sole practice of law and had served as general counsel to industry trade associations, including the National Association of Printing Ink M...