The final part of the electronics modification was to add a new display. The old display did not only have a very low contrast and some “striping”, but also had an unknown display driver and I couldn't be bothered to try to reverse engineer the communication to guess which controller is being used. 

After searching for a long time I found the perfect replacement display, the REX012864GW OLED Chip on glass module. As this OLED module required 12V-13V and also had a 0.5mm pitch cable I made a breakout board, see here for details:https://hackaday.io/project/173753-oled-breakout-board-with-dc-dc-step-up .

I removed the old display and backlight module from the clear plastic display holder and attached the new OLED module using double sided tape.

The OLED breakout PCB fits perfectly within the limited space available and is connected to the Itsybitsy board as follows: 

Connections:

I added some double sided tape on the backside of the breakout board and placed it inside the unit.

The cable chaos is complete...but working :)

I wrote some simple code that output the knob values on the display. For the final firmware I will use a scalable font and make sure there is some clearance between the text and the edges of the display window. But for now this works.