What should i replace on my high mileage vw jetta?

The catalytic converter may need replacing - that will cause a lack of power. The "clanking" noise is obviously hard to diagnose w/out seeing the car..look around and see if there's anything metal that is loose or hitting on something. Fuel cleaner in the fuel tank will help keep the carb/injectors clean. The fuel filter (or filters) may need replacing also - sometimes one near the inlet line for the fuel and/or near the fuel tank. New plugs are a good idea, if not done already