On my Gen1 Wee I bought an aftermarket corrected speedometer faceplate sold here
RacerATB He has a tutorial on how to do the faceplate swap and you do have to pull the instrument panel to get to it, so it is not the easiest upgrade but doable if you wrench. On the Gen1 the speedo and the odometer are independent so once it is fixed it is fixed and changing front sprocket size has no effect.
I now have a Gen2 Wee and I was running with a 16T front sprocket which made the speedo accurate but the odometer read low. I recently switched back to the OEM 15T sprocket and now the odometer is accurate but the speedo reads wildly high which I hate. I'm probably going to upgrade to one of the electronic fixes since the speedo and odo are coupled on the Gen2 and both are impacted by sprocket changes.
The faceplate fix works great on the Gen1. What year is your bike?