Struggling with a plummeting stock price and a decline in revenues, Six Flags Entertainment Corp. named a new CEO Monday, weeks after company officials suggested they would sell more underperforming theme parks.

Six Flags announced John Reilly, a veteran theme park operator, as its new president and CEO. He had served as an interim CEO and chief operating officer at SeaWorld Parks and Entertainment in the past.

Reilly is taking the reins of the struggling Charlotte, N.C.-based company that operates Knott’s Berry Farm in Buena Park and Six Flags Magic Mountain in Valencia.

“He’s got his work cut out for him,” said Martin Lewison, associate professor of business management for Farmingdale State College in New York, who is also a Six Flags shareholder.

Since its merger with Cedar Fair En

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