President Reagan declared the Mississippi coast ravaged by Hurricane Elena a disaster area, making the thousands of persons who lost homes and businesses in the 125-m.p.h. storm eligible for federal aid. The Federal Emergency Management Agency reported that the Labor Day storm damaged or destroyed 3,790 dwellings and 1,400 businesses in the three counties along Mississippi’s 80-mile Gulf Coast. The agency said 3,000 homes were damaged so severely they are uninhabitable and temporary housing must be found for the occupants. Homeowners may borrow up to $100,000 for repairs for businesses and up to $20,000 for personal property.