I have one of those, only it's been repowered, but, you have to drop the truck. then seperate the one piece side frame/bottom cover from the upper part of the truck, which will make changing the coupler accessible.
To drop the truck from the frame, there is a "C" shaped clip that you remove. You need the shell off, of course.
HOSeeker doesn't have the FP45, but the diagram for the GP18 is very close to the same, or at least to the FP45 I have, as far as how the trucks mount on the frame.
Check your PM.
Mike.
My You Tube
If it is like one I had, the coupler draft gear box is attached to the trucks, and thus swivels. Are you thinking of retaining that feature, or changing to body mounted couplers (which will mean that it will need broader curves most likely). You'd need to build up a pad to mount draft gear box on the body shell if you go that route.
Dave Nelson
I recently bought an AHM FP45, with a broken rear coupler. I can see no way to get in to the coupler box to put in a new coupler. Can anyone tell me how to replace a coupler in that locomotive?