LFO control for dr. who ring modulator

[dr.benway]

Just built the following which is a really noce take on the hemmo/escobedo 567 circuit.

http://www.diystompboxes.com/smfforum/index.php?topic=44298.0

There's an LDR in the circuit which adds a nice twist. Does anyone know what it would take to add an LFO control to the circuit?