I recently busted one of my old whatever-2001-something-idk-I-got-it-when-I-was-11 edition controllers and got a black smash 4 one to replace it and it felt a little off. I couldn't put my finger on it, but I kind of played around with it a bit, and let my friends use it for a few matches and they came to the same conclusion. They're just lower quality for whatever reason. They're kind of clunky, and honestly it almost felt a little loose or like... almost a 3D printed feel. A lot of my tech was ever slightly thrown off with it. Finally decided to get a Japanese white one and it's amazing. Idk, the sm4sh ones feel like a bunch of pieces of plastic (which they are, to be fair), the white Japanese ones feel so much smoother.
As far as just using the circuit board, it may work. I've taken mine apart and switched out buttons from my old controller to not much avail, but I haven't switched the shell or anything, so maybe. Worth a shot.
If it doesn't work though, it's worth the extra few bucks to get the Japanese one imo.