wouldnt be too difficult to replacement one especially because you are just replacing the shell. If you ever need help, you can always go to youtube and find a video tutorial; I always find those helpful when I made a pandora battery.
Also, they do sell official nintendo DS shells online? I was lookin for a lite one, yet I didn't see any. As for the regular kind, they are real, unforunately. At one point, I was thinkin of buyin a fat DS with one of those common broken hinges and ordered one I thought was real. It was, however, a knockoff (not to be confused with a third party one) as in it tries it best to look like the actual shell with stickers and all, but the plastic is made of a cheaper quality and the shell isn't painted (I got the red "mario kart" bootleg, which the real one is painted with a metallic paint.)