Well, liinked on that page is a circuit that approximates the Korg35 filter. It's not a very good filter, with pretty poor frequency tracking and a fairly variable resonance characteristic as you sweep the cutoff. The distinctive sound is caused by the input voltage affecting the biasing of the two transistors used as the voltage-control element in the filter network, which basically modulates the cutoff with the input signal. The filter VCO mod on CEM3394 does the same job. The Gakken SX-150 filter is broadly similar, it's worth noting. I think it's 12dB/octave where the Korg one uses two filter circuits in series to give 24dB/octave response.
Maybe something like an Arduino could be used to do the oscillator and envelopes, and then an analogue VCF/VCA circuit to process the sound...