Hello everyone...my first post here and its pretty demanding I think

A couple of weeks ago I took the Quest 1000 after a very long time. I was doing some plinking to get the gun zeroed when I noticed that it was shooting all over the place. For example. 3-4 inch groups at 20 yards Shocked When I was shooting this at the beginning of the year the gun would shoot under an inch at the same distance with ease. So last night I took the gun apart and what do I see is a broken spring.

Now since I have the gun apart, I wanted to know what kind of spring should I replace this with and also what kinda piston....I am new to this and also this is my first gun that I have taken apart so excuse my noobness (is that even a word?)
Also, below in the second and third picture, you will see the piston and notice one of the lip area is kinda worn down....seems kinda flat and also has a black spot for some odd reason.



btw, how do I take off this piston to replace it with another one? I am having trouble taking the piston off obviously so please advise on how to take this off
In addition, I also want to mention that you will see in the pic below that there is a little whole for a screw that is visible. This is where the screw for the scope stop was sticking inwards towards the chamber. I want to know how I can smooth out this spot. Also, when I was taking out the piston guide ( I think that is what its called), the part with the piston on it, it took a lot of effort to take out. I thought it was suppose to be smoother than that. What can I do to make it smooth?

BTW, the gun was pretty dry except for the rear guide and the spring which had some sort of oil that I wiped off. Other than that, the gun was pretty dry: the cylinder walls in which the front guide or piston moves back and forth was bone dry and had a few light scratches (how can I smooth these out?)
You will see more pictures below and thus my question is that what materials do I need to get this job done
1) New Spring
2) New Piston
3) Moly or Tar (are they the same thing?)
4)

?? What else do I need I am kinda blank

What is that thing called inside this cylinder?

I know that I have asked MANY questions but please understand that I want to get this gun up and running and would REALLY REALLY appreciate the help