Any city acquiring a water supply system shall have the right to supply water, any city acquiring a sewage disposal system shall have the right to supply sewage disposal, and any city acquiring a combined water supply and sewage disposal system, shall have the right to supply water and sewage disposal, to the same area to which the metropolitan district had the right to furnish such services on the effective date of the purchase contract, and for the purpose of furnishing such services such city may improve, enlarge, extend, maintain and operate such system or systems within and/or without its corporate limits: Provided, That the city shall comply with any constitutional requirements in respect to the exercise outside its boundaries of any such rights which shall not have been constitutionally transferred to it pursuant to this act.