While reverse engineering the mainboard PCB, I've been thinking about how to achieve my first objective that is to change the display for a bigger one. After a few measurements, some research and some considering I came up with the following requirements:

1. should be between 3.5 and 4 inches
2. shouldn't consume more than the actual display
3. shouldn't be too expensive
4. should display the video rapidly after powering on

I then tried to list all available solutions I could think of:

• car monitor with AV input (like those cheap ones sold on Aliexpress)
• small FPV monitor
• monitor removed from a product (PSP, another video intercom, ...)

Having one of those cheap car monitor, I tried it but the video would turn completetly white just a few seconds after appearing on the display. Also, it took a few seconds for the video to be displayed and the power consumption was around 180mA, almost double the one of the actual display.

So I threw away the car monitor idea and knowing too less about FPV monitors, I focused on finding a product from which I could harvest a display module. First thing I thought was: "Find another old video intercom with a display of proper size". So I went on mercari and Yahoo Auctions and started to look for video intercoms under 2500 yens. I created a simple list (brand, model, year, functionnal/broken, price) and then looked for each item specifications in order to get details about the display.

And I ended up buying for 820 yens this Aiphone broken video intercom with a 4 inch display. 

When creating my list I noticed that old video intercom would tend to be quite thick, surely because they use display module. This one was about 4cm thick, almost as thick as mine and sure enough:

A separate PCB with 3 potentiometers was connected to the middle connector of this display so I searched for the datasheet of the closest IC to this connector (IR3Y29BM made by SHARP) and confirmed the following connections:

Finally, after confirming that it consumes less than 90mA I installed it instead of the actual 2.5 inch one and tested it: video appeared instantly and even without connecting the separate PCB (the one with 3 potentiometers) it worked fine.

Only downside, this 4 inch display module looks like it is not gonna fit in the actual video intercom... Might need to look for a 3.5 inch instead...