Heads Up --- I have a Lionel Southern SD40 engine that came out in 2017 with the "Kinematic" floating couplers / pilot assembly. After reading this thread about the screws coming loose or missing, I pulled the engine off the layout to check it. I believe it has the same type coupler / pilot assembly as the SD45.
Sure enough, there were loose screws and one missing screw. The coupler / pilot assembly is held to the bottom of the frame with 4 screws; on one end, a screw was missing but the other 3 were tight; on the other end of the engine, all 4 screws were there, but two were loose. Fortunately I found (in my collection of extra screws) a suitable replacement screw and installed it.
I also checked the small plates attached to the main coupler assemble. They are there to keep the coupler wires in place. There are two small plates on each end, held on my 2 screws each. On one end, 2 of the screws were loose; and other the other end, 1 screw was loose. I tighten all of the screws on the engine I could find. It takes a very small Philips head screw driver, almost a jewelers size.
Fortunately, I have not had any problem with this engine, but if more of the screws had worked loose and fallen out, the whole couple assembly could have failed.