Some K-Line engines(EP-5's and GG-1's) and passenger cars(heavy weight series, 3 axle trucks) suffered very much from "zinc pest" as the foreign countries didn't filter their zinc properly or were too cheap to care.
You might be able to modify a Lionel part to use with the K-Line engine.
Don't know about the Williams EP-5 but the Williams engines I have all run very well. Only problem is that when you buy the Williams engine or passenger cars they now belong to you for life(nobody wants to buy a Williams), at least that is my experience here in southeast Florida.
Lee Fritz