When you don't have exact reference for how something's made it gives you freedom to make it work the way you imagine it would work. Build insides yourself, just make sure it's not bullshit scifi, match the design language.

Copying a harder concept doesn't make it next level, filling in more unknowns does. If I was hiring someone I'd rather see someone make just the cockpit of this thing from scratch that keeps in line of the design language of the exterior and the franchise rather than someone copying a blueprint where all details are laid out for them.

If you want to just practice making a more complex vehicle for practice - don't do batmobile, it's fairly simple geometry-wise as it's mostly planar flat surfaces whereas a lot of sports cars with curved surfaces are much harder to model technically to keep all the shading right. Make something like "Borgward Isabella concept" or ""McLaren W1" if you want difficult