EW Clara sound mod

What mod specifically are you referring to?

That's a pretty interesting site.  He is taking some of volume VCA output to modulate one of the timbre controls which if done right is something that could really improve the flat sound of the EW.  He is also using inductor coupling to alter wave shaping as a function of pitch - sort of hit or miss to make it work but if you like the sound that he demonstrates then he has been successful.

What you are looking at is not so much a mod that someone can build for you but is instead a series of somewhat invasive alterations made to an existing EW main board.  This is the kind of thing that is probably best done in incremental steps yourself so that you can evaluate each change and back out if something goes wrong.  There is nothing difficult here, but you have to be adventurous and you need to be comfortable with soldering and working on circuit boards.  If you aren't, you can learn.  It's probably not the sort of thing that someone is going to want to do for you.  And of course it would be a whole lot easier  if you had a spare main board on hand that you can purchase (or could in the past, anyway) from Moog for around $70 instead of hacking on your present board.  You would pay more than that for the mods and shipping anyway if you were able to find someone willing.

He documents his work quite well and it's worth studying his modifications some more.  I have a spare board that I might like to try it on.

   
The presented EWS modifications (like all of the other stuff) are initially intended for people who are willing to experiment themselves to enable them to do so and provide the needed starting points.
In my research I try to get as close as possible to the behavior of the early instruments build by Theremin. The described solution by changing the position of the (partial) oscillator coils is a convenient way (at least for me) to adjust the coupling/compensation between the oscillators to achieve the correct audio-waveform (with appropriate pulse to pause ratio, which mainly determines the sound spectrum) to be processed by the following limiter amplifier. This of course influences the pitch range and linearity, too. The small amount of volume control voltage fed to the VCA alters the pulse width and therefore the spectrum only very slightly.