The way I would approach this is try to find a LCD module with a similar resolution (256x64 ?) that fits physically where the current non standard module is located.
I would then interface a microcontroller between the microcontroller of the head unit and the new LCD screen to perform translation of the commands.
Depending on how close the protocol here, it could be simple or harder, it really depends. But given the high processing power and speed of today's microcontrollers, I don't expect it will be a major issue.
So, scout AliExpress for a proper LCD screen (in term of form factor). Prefer the LCD ones over I2C as they are faster and this will help with the translation.
Then, study the datasheet of the original controller, learn its command set. Get a logic analyzer like a Saleae Logic 8 (if it's a serial interfae, or a Logic 16 (more expensive...) if it's a parallel interface, and look at what the head unit CPU is doing. Try to map the signals to the datasheet and understand what it does. Then build up from here.
Depending on your experience it can take you a weekend or a month of work, can't tell, but it's definitely doable in my opinion, given that you have the datasheet of the original controller and that the resolution is pretty low.\
PS: Found this :
https://www.aliexpress.com/item/32833686161.htmlNot a direct compatible option clearly, but it seems like it is smaller than your screen so it should fit. Ultimately you want to make a PCB for it, with the microcontroller on it to do the conversion, and put that in your radio.
First step :
Get familiar with the controller of the original screen and hook up a logic analyzer. See the instructions and data flowing, understand it.
Second step :
Find alternatives on eBay/AliExpress like the one I show. Study the controllers, look at how they work. See how you can adapt the data (probably ignore the original init commands going to the original controller, and send hardcoded init commands to the new controller, then simply transfer pixel data)
Third step :
Make a prototype PCB to solder the new LCD/OLED display. Connect a microcontroller to it and display Hello World.
Then connect your prototype PCB instead of the original controller to the head unit microcontroller (Don't fry it ! careful !)
Then start working in translating the data
Fourth step:
It works ? Congrats, now make a small physical PCB to mount the new display in place of the old one, with the microcontroller on it and the pads where necessary to make assembly easy
Something like this. Definitely a project, BUT if you do it well, you can even think about selling it, if there is demand for this. Good luck !