A FMG can only become a PA, if they go to PA school. There is no "challenging the boards", similarly, a US medical grad, can't become a PA either, (not they want to)they would have to go through a PA education also.
I have heard of a case where a physician who is an FMG, who was licensed to practice medicine in the US lost their license due to leaving the country, and instead of going through the hassle of US medical school, or residency, or whatever it takes to get relicensed, they option to just go to PA school, and work as a PA. If they get back into their previous specialty, and work with a physician they know, I would guess only surgery would be out of their scope of practice.
(the example in question is a 55 year old french urologist, who returned to france to care for an ailing father, and lost his license during his time away, he is currently a 1st year PA student at a program in Pennsylvania)