As I see it, there are 4 places where this can be done:-
At the input to the gearbox
At the output of the gearbox
At the final drive
By changing the wheel diameter
All involve a fair bit of ironmongery. Personally, I reckon the rear wheel/tyre could be an easier option than making new gear sets and altering gearcase clearances and oil feeds.
But you have to find a bigger motrorcycle tyre first, unless you go to the darkside...