The cheapest way to stick a fully featured computer with a screen in a prop is to re-use an old mobile phone and I happen to have a Motorola Moto G4+ which I replaced a few years back. It's still a very usable Android device and mostly I retired it due to charging/battery issues.
For it to not just be a phone in a case, it needs a keypad and other connections so I started messing around with an M5 Stack card keyboard I have.
The action is pretty nasty but I persevered and ended up with a 3D printable holder and levers to which I tried to add keycaps.
The staggered triangular keycaps were a deliberate Blake's 7-esque choice that don't really work without a surround of some kind.
With a game that could benefit from me having a 'cyberdeck' happening in a week I dusted off the idea again and suddenly remembered I have some old Xbox360 chatpads in a box.
Information on hacking these into other devices is generally very old and often a sea of dead/obsolete links, but I quickly found this video that gives you just enough information to make it work. The example Arduino sketch worked immediately which I call a win.
After fitting the trackball there should still be some pins over on the Arduino. So I might wire them up to some LEDs for blinkenlights, or perhaps to have some extra buttons.