The LCD on the back cannot be replaced with a different or larger one, and it is not possible to move the system into a different body shell that has a larger screen, as the circuit board/system is not designed for a larger screen.
The D80 I know didn’t have the ability to output to an external screen live, so doubt the D70 had it, as that would be the only other option if you need the screen larger.
A new body would be better, as there is other improvements to the other parts of the body other than the screen.