We have what I think is the original strainer which also acts as a plug and is rotated to allow drainage. The one in the head basin is similar but smaller. Without measuring, looks like about 2.5 inches across. Not long ago I had to do a s/steel wire winding to re-secure the plastic knob on the shaft.
I think you just have to measure up and visit your hardware store to get separate strainer and plug.
Island Gypsy 36 Europa "Doriana"