Shower Bar Valve Repair or Replacement

Shower handle to turn shower on or off is now stiff and difficult to move. Potentially limescale build up or wear and tear. Looking for help either repairing the existing shower system (bar valve/thermostatic mechanism) or to replace it completely with a new unit depending on cost/complexity. Shower unit model has been discontinued: Hansgrohe - Ecostat Shower thermostat 1001 SL for exposed installation