You're looking at about $180 for a recalibration from memory. This only recalibrates your speedo though, odometer will still be off.
You're best off changing the speedo drive in the gearbox. There's two gears inside which you need to mix and play with from other gearboxes to get your correct ratio.
Gears from a K50 can be used if memory serves me right.
Do the above or just get larger/smaller diameter tyres and fix it up the easy way.
I'm personally just driving it 20% wrong, reading too quick. 120kmh is really 100kmh.