I haven't looked anything up in the service manual
yet, but it sounds to me like the display is not communicating with the ECU or anything else. Since the history of this 420 is unknown to you it might be a good idea to verify first that it has the correct display, harnesses and ECU on it and then begin to troubleshoot their connections to each other. The wiring harnesses and connectors must be undamaged. The connectors must be clean with no bent or dirty/corroded pins. The frame grounds from the harnesses must be clean and tight, etc.
You'll have to take the plastics off while you work on it and you'll need a decent quality multimeter. Use the service manual
for reference and begin to check for continuity in the power, ground and data lines between the ECU and display. It may be as simple as a bad connection in a plug or it could be much worse. Once you find it the display and ECU should begin to communicate with each other and some basic functionality should return. Only then will you be able to begin a more thorough diagnosis of any preexisting faults.
Hopefully the ECU has not become damaged or voltage spiked yet. Always turn the key off before you disconnect or reconnect any plugs or components under test to avoid spiking the ECU.
Welcome to the forums!