In 2006 Suzuki put out a Bandit with the WC/FI motor that appeared to have the MkII frame. Of course I have no idea what the subframe of that 2006 looked like, but I'd be surprised if it were completely
Make a friend with a 1250, buy a case of beer and spend a Saturday seeing if the pointy tail section of the 1250 fits on a MkII 1200. (My guess is that the subframes are nearly equal, but the crucial bosses and brackets are different.)
Now if you have a MkI 1200, the 1250 body parts will certainly not bolt right up.
That said, after a little angle grinder, sawzall and welding you can bolt up anything you want!