I would try shielding the Clav and not change parts in the Wurli to retain its tone.
What year is your Clav? Is it a D6?
By around 1977, Hohner released a model that was shielded to address the noise problem.
So let's say the Clav is the issue not the Wurli.
But there are work arounds by shielding the Clav's amp.
This does not involve using "hiss" caps which ruin the classic Clav tone.
A copper box is made and connected to ground.
Shield the electronics or get a keyboard stand to lift the Clav further away from the Wurli.
May not be ideal but back when these were made no one thought about players stacking them one on top of the other.



