Fast Square Wave Oscillator

Preamble: Had an old unreliable TR-2 laying around and started experimenting on it. Once change was reducing the rate limiting resistor so now the Rate knob can put the circuit into ring mod territory with Wave set fully to square and Depth set to max (video). Which is extremely cool, and I'd like to try to recreate this effect as simply as possible. the THAT2182 it uses is prohibitively expensive for DIY so I'd like to use a jfet (figure 7.1b) as a jumping off point for the VCA, but I'm still early in the learning curve for side chain stuff like envelopes and LFOs.

TLDR: What circuits or building blocks would be useful as a fast square wave oscillator to control a VCA?