I have that engine as well. I believe you are talking about the stud, spring and clip assembly that goes in the middle of the truck, right? This is it on mine viewed from underneath and the side:
There is also a small washerhead screw at the front of the truck but as that has no spring I think you mean the set of parts that is actually fitted through the drawbar.
While the Lionel replacement parts list for this first run Legacy FEF-3 doesn't appear to me to list these items (although it does list other screws and fasteners, of which I have got literally scores from Lionel), the second run Legacy FEF-3 parts list does include them and those models in most respects use the exact same tooling as the earlier ones. So if you look, for example, on the SKU 6-82806 parts page instead of 6-11116, this is the exploded parts illustration and the stud, clips and spring are numbered 7 and 34-38 shown on the lower right of the image:
These should fit but you could always ask Lionel to confirm.
Incidentally, I don't see how those parts could come out (maybe because a clip broke) without leaving some residue on your layout, so they might be in between track ties or something. Also I don't know how you re-install this assembly and it might mean taking the boiler shell off but that's relatively easy on these engines. I caution about the handrails at the very front which fit into two small upright tubes on either side of the pilot (part 20 in the image). These require some care to get them back in place when you are re-assembling the engine.