Upstate New York’s embattled Saratoga Performing Arts Center has chosen a well-connected state Senate aid to replace longtime President Herb Chesbrough, who is leaving under a cloud after a scathing report took the center to task for its shoddy management practices. Marcia White, who will earn less than half the annual salary that Chesbrough enjoyed as president, will take over the running of SPAC in March, and plans to spend her first few months developing a new business plan for the center.