Developers for University Station  and town planners will prepare a new zoning overlay district for the 2-million-square-foot, mixed-use development  in time for a vote at a Special Town Meeting in December.  That work will follow a primary review of the proposal. While the 130-acre site  adjacent to the Route 128 Amtrak and MBTA station is currently zoned for mixed uses, the zoning does not factor in all elements of the plan being proposed by New England Development,  Eastern Development,  National Development/Charles River Realty Investors,  and Clarion Partners,  town officials have said. The project that replaces the failed Westwood Station  would include 550,000 square feet of retail space,  500 to 600 residential ownership units,  150 to 200 housing units for senior citizens,  a 150- to 175-room hotel,  and 300,000 to 400,000 square feet of office space,  according to a preliminary description. It is roughly half the size of the former project located near the junction of Interstate 95 and Route 128.  If residents support the flexible zoning plan, shovels could hit the ground next spring with some phases of development open for business by 2014.