Tom,

if you can get the first one or two threaded sprockets off, you cna check whether the others are threaded or splined. Once you have the model and exactly what you need check at yellow jersey in Madison, Wisc. They keep lots of loose sprockets for building custom freewheels and might have the Shimanos you need. If you ever need Sun Tour Winner series, or Sram freewheel sprockets, I still have a bunch in most but not all sizes.