By Svea Herbst-Bayliss NEW YORK (Reuters) -Activist investor Jana Partners has teamed up with football star Travis Kelce to push Six Flags Entertainment to improve its marketing and customer experiences, the investor said, after disclosing a 9% stake in the company. Jana Partners' Scott Ostfeld announced the position at the 13D Monitor Active Passive-Investment Summit in New York on Tuesday, saying he has partnered with Kelce and two industry executives to pressure the company to improve its performance. Shares of Six Flags shot up 17% on Tuesday after Ostfeld spoke to $25.63. The stock is down 58% since it completed a merger with fellow theme park operator Cedar Fair in July 2024. Six Flags has a market capitalization of roughly $2.6 billion, and has struggled this year as rainy weather a

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