I KING OF THE FABULOUS FAKES KENNETH JAY LANE HAS COME A LONG WAY FROM HIS MIDDLE CLASS JEWISH BACKGROUND - TO HEAD HIS OWN COSTUME JEWELRY COMPANY AND SHOPS. BY MAIDA PORTNOY 50 IN STYLE t's lunchtime on a furnace-hot summer Monday in New York. If he takes the time, lunch for costume jewelry designer Kenneth Jay Lane means Mortimer's where lineage counts more for table requests than time of arrival. More than just a place to satisfy your hunger, it's sort of a club for Lane and the other names one reads about in Women's Wear Daily. Seated at a coveted table near the front door, Lane sips a glass of Lillet and smokes one of his ever-present cigarettes, coughing intermittently. He converses with luncheon companions, never missing a beat when it comes to greeting friends and customers. In Lane's case, the words 'friends' and 'customers' are often interchangeable. Patrons with surnames like Burden, Phipps, Boulandris and Guest acknowledge "Kenny" with a warm hello or the continental both cheeks kiss. Conspicuous by their absence, the majority of regulars are off vacationing at summer residences. Lane himself is usually in Bodrum, Turkey this time of year with his friends, the Ahmet Erteguns. Most days they lunch aboard the New York financier's boat and their group of cronies share, as Lane describes it, "lots of giggles." But business has kept the 55-year-old designer at home. Business and a projected 1987 gross of $25 million. Just when it seems he could taper his work schedule a bit, he works harder than ever. His jewelry, with an inventory of over 4,000 designs, is sold throughout the world in specialty and Simulated coral and diamond necklace and earrings are based on a design created especially for Jackie Onassis.