Ultrasystems, an Irvine-based engineering and construction firm, and Hadson, an Oklahoma City natural gas producer, have reached a definitive agreement in their previously announced merger plan, under which Hadson will acquire Ultrasystems for $89 million in stock and cash. The deal, which is subject to approval of Ultrasystems' shareholders, is expected to be closed in the first quarter of 1988, the companies said. Under terms of the agreement, Ultrasystems shareholders will receive 2.35 shares of newly issued Hadson common stock and $1.50 in cash for each of their Ultrasystems shares. Ultrasystems has 8.9 million shares outstanding. Ultrasystems stock closed Wednesday at $7.25 per share, up 25 cents, on the American Stock Exchange. Hadson stock closed at $4.25, down 15 cents, on the over-the-counter market. Officials for both companies have said they anticipate no layoffs among Ultrasystems' 1,300 employees.