My client has decided for the moment to not have an OLED display installed in this vintage synthesizer service/restoration project, given the expense that would be required in burning new firmware with a hacked operating system to adapt to the single line write of modern Hitachi-compatible display controllers.
We've agreed to just put a replacement EL backlight under the display to replace the faded-out original.
It's not quite as bright as he would like for dark stages, the original probably never was either. Wondering if there's a quick mod that could be done to the EL backlight HV inverter circuit to increase the frequency and brightness a little bit.
Schematic (HV inverter is at top left):
The tantalum has been replaced with a regular electrolytic of the same value.