What I would try is replacing the 150K resistor next to the output with a A250K volume pot and connect your output to lug 2. You then adjust it to reduce the output. Note, however, that like a Klon, when the effect is off, your signal still passes through the JFET to the output, but at near unity gain. The volume control would then make your signal less than unity gain. Maybe you could use a DPDT switch to bypass the volume control when you turn it off.
Here's the schematic: