Definitely just buy a handfull of jets!! Get a few new plugs and start your trial and error in your back yard or if you don't have lots of land, somewhere that you can ride around and open it up a bit and then keep your tools all in one spot to keep changing out the jets as needed. Don't dump the gas on the ground either!!
For tires, in the woods, I would recommend atleast 21 (which is stock) if not 22" tires. For hill climbs I would suggest a lengthened swingarm. I'm a big fan of 22" tires in the woods!! Also, good skids are a must!