The Quantum Distortion is a clone of the Ibanez MT-10 Mostortion MOS-FET Distortion, a fairly obscure pedal in Ibanez’s “10 series”. The MOSFET op amp that gives this pedal its name, the CA3260, is obsolete as well. This op amp is the only MOSFET component in the pedal, and there are no suitable current-production substitutes, so if you’re building a clone, you’ll have to hop on eBay and find one.
That said, the MOSFET op amp isn’t the only thing unique about this circuit and it’s still worth building even if you can’t find one—it just won’t be a MOSFET distortion. It will still have its own character whether you use the CA3260 or a JRC4558 or anything else, and who knows—you may find you like one of those better!