New set of tires rather than OEM.
After-market seat rather than the OEM.
Adjustable rear-sets rather than the OEM.
Engine guard or sliders.
Handlebars.
Shocks.
Exhaust.
Windshield.
Where would we be if the wheel never got invented? Oh, where would we be if nobody thought of using two wheels in-line and ride in between them? Oh--- and where would we be if nobody thought of using rubber for the wheels..... Oh and where would we be if nobody thought of blowing air in the tire...