Link:
http://www.bartonmusicalcircuits.com/4046/
Looks interesting, your version has four frequencies? (Original has 7.) It’d be interesting to have outputs for each frequency separately, then feed them into a VCA mixer to mix them under voltage control.
Edit to add: I just realized since this thing generates square waves, could you use an analog switch as a VCA? Maybe not a MAX313 but maybe a DG412? Actually two. That’d give the same functionality but with voltage control, or at least I’m guessing it’d work. Others would know better.