MorganS:
I think the easiest way to do it will be to rip out all the internals and put a Micro, Nano or Mini Arduino in there. With a screen you buy off-the-shelf you will get the information required to interface to it.Since I've made a lot of projects with custom circuit boards, I would make my own PCB which holds the buttons. You will probably find that the buttons are relatively simple contacts or maybe you can put a modern tac switch under each one.
Thanks a lot for your help.
I've decided to go with your idea to put an arduino micro inside the phone and will use a nokia 5110 display and a simple of the shelf 4 button pcb with 1 button cut off to fit the housing.