First off, to answer another question you had about the right side cylinder base oil seep - no - dropping the bike on the other side didn't cause that to happen. Especially having dropped it on the other side would have affected the left side, not the right.
As for the speedo - it's possible the drop helped dislodge a certain pot metal gear that tends to come loose anyway. The gear commonly looses its grip on its shaft and spins freely and slides to the side causing all sorts of issues. I frankly consider the odometer issue to be the most common failure on these bikes - from 1974 to 1995.
Repairing these things isn't an easy job, and made more difficult on the R65 by the crimped-on bezel. Once inside, it's a gear between the odometer and trip meter, which requires the total dismantling of the gauge - needle, face, and everything else - to access.
Some guys have JB Welded the gear with success. I've also repaired guages that were JB Welded and didn't stick. If you do attempt that fix, be sure to get the surfaces very clean. If you're careful, you may be able to JBWeld it without dismantling the mechanism.
Good luck! And let us know how it works out.