The most difficult part of replacing a clock movement for most people, is just measuring the length of the hand shaft. These two measurements are the most important measurements you have to take. You check the overall length of the hand shaft (A) and the length of the threaded part (B), where the hex nut goes. It’s really not difficult to do.
Clock movements are normally attached to a clock dial or other surface from the back side. The thickness of whatever the movement is mounted to will determine the shaft length needed. To check this, you must have the clock movement removed from the clock case. Remove the clock hands and check for a small brass nut that would be under the hour hand and holds the movement in place.