Please note that in each category, I list items in increasing order of severity... from least costly (or least extreme) to most.
Keep in mind that each of the pages shown below have a series of mods from simple to very involved. When diddling about with changes to your motorcycle, try to keep in mind that each system affects others. Stronger brakes may upset the suspension; changes to the intake or exhaust need to complement each other.
Comments are intertwined about effectiveness. I sell all this stuff from my shop California Cycleworks.
Be sure to check out Zina's Monster modification page.