I replaced the spring in one of mine recently.
If I remember correctly, there is a pin in one of the handles that, when the pin is drifted out, allows the handle to be taken from the shaft. That part is easy. Take photos for reassembly!
Now the spring. It is a "torsion spring" made from 2 mm diameter spring steel. To make one copy the old broken one. I am in the Ionion Islands and spring steel is not available. I used the key ring type holder from a set of Allen Keys.