To tell you the truth I'm not sure WTF is beneath the Electric XII's pickguard in terms of pickup and control-cavity routs. But as can be seen in the photos below, the overall body shape externally does resemble the Jaguar/Jazzmaster platform. If you start with a "raw" body blank and can do your own routing you'll likely be able to pull this off. However the monkey in the wrench may lie in locating the hardware and electronics specific to the model. I don't know that Fender ever re-issued the original 12-string so the options for parts may be limited.