Many iPad models featuring the Lightning connector allow you to connect an external display over HDMI using a Lightning Digital AV Adapter. Recent iPad models featuring USB-C or Thunderbolt connectors allows you to connect a display without using an adapter.
As for the touch digitizer, it is possible to connect it using the USB protocol to for example a recent iPad having a USB-C connector. Realistically you want to use a USB-C dongle/dock that allows you to have USB in (for the touch digitizer), DisplayPort/HDMI out (for the external display) and power in (for charging the iPad).
You could create a custom case that holds the iPad, has the cable connected to the iPad, an adapter/dongle/dock, and an external screen featuring an appropriate interface (HDMI, USB-C/DisplayPort, depending on iPad model) and USB touch-digitizer. Such a custom case would most probably be quite large.
You cannot connect a larger screen to the internal display connector inside the iPad, unless that larger screen has been made explicitly for the purpose of being connected to that connector. As far as I know, such a display does not exist. Note that even such a larger display could only be physically larger - it cannot have a larger resolution than the original iPad display.