Lukelaw is correct. These engines are geared to deliver 4.77mphy/1000rpm, in top gear. Unless you're an ultra-lightweight rider or/and, every road in your locale has a descending grade + strong tailwind, your engine will run out of torque before gearing can reach 5.0mph/1000rpm. And, a stock K0 will run out of breath around 9000rpm...as evidenced by typical top speeds in the low 40s. The 4-speed, H-models, tend to do a bit better; advancing the ignition lead seems to extend the power curve by ~1000rpm.
HP = MPH
There's no way around this, it's basic physics...and reliable enough to be used as "the poor man's dyno". And, airflow = HP potential. So, you have to increase airflow and your choices are: increased displacement (more air volume per cycle), increased rpm (more cycles per unit time), or a combination of both. Leaving everything beneath the head alone, that leaves: hotter cam, portwork, larger carb, ignition. The simplest mods: SL70 (a.k.a. "fast road) cam, set the point gap at the practical limit (~0.018-0.020") to advance the timing. If you go any wider, the points won't close. Remove the cone restrictor from the inlet side airbox boot. Beyond this, life gets more complicated...port work, hotter cam with matched valve springs, CDI conversion to get spark advance and an aftermarket 18mm carb. Gearing will remain constant, for any of these tunes. The only parameter that will change is the power peak...you're looking for another 1000-1500rpm.