YOKOSUKA, Japan (Reuters) -When Nissan's Oppama plant opened in 1961, it was one of Japan's first large-scale auto factories and a symbol of the company's global ambitions. Sixty-four years and millions of cars later, the storied plant now faces possible closure as Nissan sinks deeper into crisis. Battered by declining sales in the United States and China, Nissan faces a mountain of debt repayment and is scrambling to upgrade its ageing line-up of vehicles.