I'd assume the electronics are very modular and probably just clip into place in a single unit. You do need a battery compartment or something though, and the buttons need to fit well. Wheel can come from donor mouse.
Unfortunately they are not. Mouse motherboards are generally shaped like the mouse, i.e. right-handed, and the side buttons are on ribbon cables too short to move to the other side. Impossible to reorient without serious work.
It seems like a massive pain though. Rebuilding a non-symmetrical mouse the other way would take electronics tinkering as well as 3d printing, and probably a ton of work to get all the connections and tolerances right.
I am left handed, and I use trackpads with my left hand, but I gave up on lefty mice years ago and use an ergonomic one right-handed.