I am learning from all of the above comments, but I have not installed my new light switch body, just yet. I have a couple of projects all going at the same time, so this one has taken a back stage at present. I purchased a couple of replacement light switch bodies in order to try to clean up at leas tone and make it workable.
The only one I really had difficulty with in joining the two pieces together was the repop one. One of the extra light switch bodies I had was full of old grease, grime and what ever inside. The outside looked as if someone had taken under coat paint and sprayed it down, but it cleaned up fairly easily. I was thinking that some Brasso may help to clean the contacts inside, but I have not tried it. Heat actually melts the grease and makes it even worse to remove. I used a vinegar and water bath and had some good luck with cleaning the parts with that. Some grease and grime is still lodged behind that spring and plastic part inside the lower portion of the light switch body, but qtips seemed to get most of it out. Like say, I have not hooked up my wiring harness just yet, so I am not sure if I made any gains or not at this point.
I have one more light switch body on order, so I can't wait to see what condition it is in and if I can make it work or not to replace the old light switch body that i had started with.
Someone else may answer your questions better as to the plastic piece and contacts on it. Someone here had some old harnesses that they had picked up for a buck at a swap meet. If they still have such, you may be able to get them to sell you a harness and use the plastic piece from it to see if that swap would be effective or not. Someone may have one they would give you if you asked. Good luck with a fix.