U.S. allies Japan and South Korea would struggle to quickly ramp up shipbuilding to meet U.S. demand for alternatives under President Donald Trump's plan to impose port fees on China-linked ships, a top Japanese shipping executive said on Monday.The Trump administration is drafting an executive order in a bid to revive domestic shipbuilding and weaken China's grip on the industry.Japanese shipbuilding is running near full capacity, with little scope for expansion until 2028, while shipbuilders in South Korea…