SWAMPSCOTT, MA -- The YMCA is replacing the North Shore’s smallest YMCA with a new, 90,000-square-foot facility with an eight-lane pool, a 7,000-square-foot gym, a 7,000-square-foot fitness center, a 6,500-square-foot gymnastics center and a satellite school for the Boston Ballet. The Y purchased 19.5 acres of land three years ago after searching for more than 20 years for the appropriate site for the new building. The Y broke ground in late September and expects to open the facility in 2008.