Call a local speedometer shop.

For instance:  http://www.paspeedo.com/

IF the cluster and speedo design is the same as my 200E, then it may be
a simple fix.

Without disassembling the cluster, look at the odometer, if it looks
like it's skewed, the right hand side lower than the left hand side,
then it has just jumped gears. All it needs (hah!) is that you take out
the instrument cluster, remove the speedometer module and push the
spindle back in, and it should engage the gears and work well again.

IMHO, it was a bad design, but it's usually due to the trip meter
getting sticky, and if you press the reset button, all the number may
not line up properly at "0000", if so, it will jam and cause pressure to
 build up on the drive spindle, which then derails and thereby
dislodging itself. Somebody should have put a friction clutch on the
trip meter drive gear.

While it's out, I would clean and lube the odometer and trip meter gears
as well.

Getting the cluster out:

http://www.mercedesshop.com/Wikka/W124InstCluster

Getting the speedo module out will require that the tacho module (on the
left in the picture:
http://www.mercedesshop.com/diy/w124inst_cluster/panel_lights.jpg
be loosened and the light dimmer control (white ceramic part) be removed
as well.

Some additional tips in this Forum Thread:

http://forum.mbworld.org/forums/showthread.php?p=1609434

Cheers,
ws

Yup.

Nearly if not all German postwar cars use the same VDO mechanism
which has as a weakness one gear that ALWAYS (eventually) fails.

http://odometergears.com sells just the gear and it's the only place
to get it. It's pretty easy to take the thing apart and replace it
and it should be good for another 20 years at least.

Some hopefully usefull instructions for getting the thing out are
here:

    http://articles.mbz.org/instrument/