The glass is bigger on a Mk2 so the metal frame is narrower. That means that if you are fitting an XR4i or Cosworth upper spoiler to a Mk2 tailgate, they will overlap the screen just a little.

The holes in the top for wiring and washer fluid are different, as are the wiring looms inside the tailgate.

Mk1 doesn't have a mounting hole for the radio aerial amplifier.

Lock and catch can be swapped over.

Mk1 hinges are welded to the tailgate. On a Mk2 they bolt on. So long as you have the Mk2 hinges they are a straight swap.