BELFAST, Maine — What should the limit be when it comes to the height of buildings in part of downtown Belfast? The answer could guide Belfast developers such as Paul Overgaag with plans to transform Washington Street, a formerly industrial area that city planners believe has a lot of potential. Overgaag, a Cambridge, Massachusetts-based restaurateur and entrepreneur who has owned property in Northport for more than a dozen years, owns two abutting properties in downtown Belfast — the Green Store building at 71 Main St.