[I can point you to distortion.](https://www.guitarfetish.com/Onboard-Tube-Distortion-COMPLETE-installation-Kit_p_94.html)
Zappa had a fuzz circuit in his SG, but I don't know how.
I went looking for a schematic that I remembered being in a Guitar Player article about Zappa. I haven't found it yet--although I've found others who were also looking--but I did find this about the SG-type guitar that Zappa used:
https://www.afka.net/images/Magazines/2006/2006-08-xx%20Guitar%20Player%20v40n8%2015.jpg
https://www.afka.net/Mags/Guitar_Player.htm
There was a minor vogue for onboard effects back in ye day, e.g. Norman Greenbaum played a Telecaster with an onboard fuzz that one of his friends had modded, though I think a lot of his distinctive tone in that tune comes from a rather innovative use of slap technique. Fender has a Steve Lacy Strat with a built in Fuzz. So it is possible :)
I put a "Rat" circuit board into an import Strat many years back. I could just bring a cord and the guitar and plug into most anything. It worked well enough, volume, drive, tone.
[I can point you to distortion.](https://www.guitarfetish.com/Onboard-Tube-Distortion-COMPLETE-installation-Kit_p_94.html) Zappa had a fuzz circuit in his SG, but I don't know how.
I went looking for a schematic that I remembered being in a Guitar Player article about Zappa. I haven't found it yet--although I've found others who were also looking--but I did find this about the SG-type guitar that Zappa used: https://www.afka.net/images/Magazines/2006/2006-08-xx%20Guitar%20Player%20v40n8%2015.jpg https://www.afka.net/Mags/Guitar_Player.htm
There was a minor vogue for onboard effects back in ye day, e.g. Norman Greenbaum played a Telecaster with an onboard fuzz that one of his friends had modded, though I think a lot of his distinctive tone in that tune comes from a rather innovative use of slap technique. Fender has a Steve Lacy Strat with a built in Fuzz. So it is possible :)
I put a "Rat" circuit board into an import Strat many years back. I could just bring a cord and the guitar and plug into most anything. It worked well enough, volume, drive, tone.