where can i buy
 

a header for my gy6? can I pull the guts from the stock pipe w/o rejetting the carb?

You can- should add a Uni-filter to help on the air coming in to go along with the better outflow.Depending on your elevation and the stock jet it came with you may not need to change jets and the open exhaust ends up being a performance mod. Performance exhaust for the Howhit/GY6 is expensive($200. range).I just made a new pipe with 1-1/8" tubing to give it a better flow and had a 250 muffler with same size pipe to keep it quiet.

There's not really any kind of actual tuned header for the gy6 on the market except for the MRP. The mrp is not tuned, way over priced, a copy of another style no longer available and geared more for bling than performance.
Simply gutting the stock muffler is the most economic route and does improve performance significantly. This gives you a exhaust that is tuned more for higher rpms and speed but is much louder. . you do need to re jet! Average jet increase is around 5 sizes. 10 sizes if you add a aftermarket filter. Stock jetting is around 114. after gutting and filter around 127.5
We make up our own pipes. If you wanna try? and you got welding and pipe bending resources. A short, small diameter (stock) makes good power in the higher rpm band and top speed. A longer, larger diameters tend to give more torque. We found a decent all round pipe is to use the stock pipe out of the head for 8" then stepping up to I" for another 10" then increase again to 1,1/4 for a total of 28" connecting to a 1,1/4" straight through muffler.