Hi,
This is a straight swap in terms of how it mounts to the box at the back. There is however a little modification required.
The fitting at the back of the 16v servo needs to be swapped with the one on the  Mk1 servo as it has a hole on it which is needed to hook the return spring into.
The spindle at the back of the 16v servo needs to be shortened so that it will be the same length as the mk1 servo.
Measure the distance from the back of the original to end of the shaft before you start to remove anything.