I'm thinking of a DMC pipe and a K&N power kit with a modified airbox, larger rear tires (for clearance and bite) and maybe one tooth smaller front sprocket. Do I need a whole pipe or just a header. What about a silencer? Is there a better set up? What size jets do I need? I'm at about 1,000 ft above sea level with hills all around. Any suggestions?

 

First, as to the header on your Warrior; unlike certain other makes the header is quite large & capable of good exhaust flow. There are some who make a/m head pipes for the Warrior, but in my opinion they serve mostly in a cosmetic capacity.
At 1000'elev you are talking basically sea level so far as jetting is concerned. I'm not sure whether the K&N "power kit" contains the filter or merely a bunch of jets. In any case, I'd forget the "kit", buy the K&N filter if that is your choice, and simply get two or three main jets @ $2 or $3 apiece. With your stock main jet of 145, I'd start your rejetting test with a 155, which is 3 sizes fatter (excluding the 147.5 which is only a 1/2 size). Then run a standard jetting test/plug read. Depending on the plug color you may have to run one size leaner on the main. I would doubt you will need to go fatter than a 155, but really, only your own engine can tell you that!
Finally, you didn't mention your riding style, but in any case I would do all the jetting first and then think hard about the tire thing. If you do decide run different sized rears I'd consider shorter (20") rather than taller for vastly improved handling. A good swing arm skid would take care of the 1"loss in ground clearance and you may even wish to run a LARGER c/s sprocket to maintain or increase gearing, again depending on your riding style.

 

The power kit contains a filter, a pre filter, and jets. I do a lot of trail riding, basically just riding dirt roads and trails no competative racing. Is the stock pipe really as good as an a/m header? Just need more speed at both ends.