Welcome!
A Wii U gamepad definitely seems like it should have a lot of space inside of it for how notoriously clunky it is, but there's really not a lot of room inside it. People who have built portables in those in the past have needed to add a spacer to the back of it to give themselves more space.
As far as using the parts already inside it, the battery is definitely unusable, and I'm undertain how useful the speakers or IR bar could be. Either way, I saw you posted in the welcome thread that you were considering building a G-Wii, and I would reccomend taking that path for a first portable.
Good luck!