I unfortunately do not have a very robust knowledge about what potential ICs are out there and if there is one for this application. Working off of this block/circuit diagram from the 90s, wondering if the VCA/VCA processor or the power supply could be swapped for ICs for cost or simplicity purposes. It's for an analog theremin (musical instrument).

 

It is quite likely that some of the circuitry can be replaced with operational amplifiers. It would take some engineering skill to do this, however.

 

You would probably be better served designing one from scratch; the core of a theremin is very simple.

 

Why bother? You have 8 transistors. You could get better performance if you used dual transistors for the differential amplifiers if you could find some that are the same piece of silicon. Most that I've looked at were two separate die in the same package which is only slightly better than two separate transistors.

 

For the transistor pairs:
CMXT2222A MMDT4401 Dual transistors will help a little.
https://www.onsemi.com/pdf/datasheet/ffb3904-d.pdf
4

 

I built a simple theremin for a demonstration.
It was based on an LM331 voltage-to-frequency converter.

 

A very different approach:
Use two distance sensors to know how far your hands are above the table.

Use some software to convert distance to tone and distance to volume.
Write the software on a Raspberry Pi.
I cannot be the first to think of that.........youtube , arduino , more Pi ,

 

The Voltage Controlled Amplifier and the VCA controller are both contained inside a dual operational transconductance amplifier. I like the transistor approach - genuine 2N3904's are kind of inexpensive, if you went to a transistor array like a CA3046 you might save a little money, there would be fewer parts to wrangle.