Looks to me lots of effort for little gain. Usually those devices are driven by a custom chip, even professionals would frown at this kind of project.
I don't know what kind of screen that GPS had, but I would go for a car rear view screen as rear view cameras have an rca connector, and is easy to plug them on the pi.
I tried it with a tiny 4.3 inch one like this Amazon.com: 4.3 Inch LCD TFT Rearview Monitor screen for Car Backup Camera: Car Electronics and it was just to plug it to the RCA and change the framebuffer resolution on the pi to have a working - but tiny - screen. This one is 17$ but the one I bought was even cheaper.
The quality is not great though, but I guess you can aim for a better rear view screen to get more quality.