Combine sugar and flour in a medium mixing bowl; stir well. Add loquats, and toss gently to coat. Place loquat mixture in a greased 10- x 6- x 2-inch baking dish; sprinkle with lemon juice. Dot with butter.
Roll pastry to 1/8-inch thickness on a lightly floured surface. Place over loquat mixture, leaving a 1/2-inch overhang around edge of dish. Turn pastry edges under; press firmly to rim of baking dish to seal, and flute. Cut slits in crust to allow steam to escape. Bake at 400° for 40 minutes or until lightly browned.