From the pics above, it looks like that "plate" is drilled and tapped to accept a screw that will hold the "draft gear box" for a coupler in place.
You'll need to ascertain WHICH SIZE/TYPE screw fits.
It might be 2/56.
It might be metric (2mm might be a good one to try).
You'll want to take the frame to a local hardware store that has a lot of screw sizes, and then "see what fits".
Kadee couplers come with a plastic draft gear box that has a good chance of fitting.
You could try the #148 whisker coupler (medium length shank with center-mounted head).
But be aware:
The coupler should be at the right height.
Take a credit card and place it on the track.
It should just clear the "curleycue" (actuator) on the Kadee coupler.
If the standard height coupler is too high or too low, Kadee also offers couplers with "over-set" or "under-set" shanks, to compensate.
If the standard shank length is too short, Kadee offers "extended" shank couplers.
These make the coupler head "stick out" further -- not exactly prototypical appearance, but it will clear the loco. Also could be good if you're running the engine on a layout with sharp curves and #4 switches.
