Usually it is the little plastic small gear at the back that drives the larger white one that simply has deteriorated with age and can easily be replaced as set off ebay.
If it is the circuit board it can be fixed by someone who repairs speedo's
Swapping things around from another parts car can be a bit more complicated
560 SEC/SEL/SE has a 260km/h calibrated speedo and face with the markings on the face appropriate for that set-up
The 420/500 SEC/SEL/SE has a 240km/h calibrated speedo and face with the markings appropriate for that set-up
Your in Dubai so i am assuming your 560SEC is possibly a 300hp version?
The standard lower output 560 engines run a 2.47 ratio diff
The 300hp 560 engines run a 2.65 ratio diff
If your coupe is an AMG modified version? it may run a lower ratio diff 2.82 or 3.06
The gen2 500 SEC/SEL/SE runs a 2.24 high ratio diff
You will need to check the rear face of the diff down on the bottom right corner at a 45deg angle the flat piece that has the diff ratio stamped into the metal (you will need to wipe it clean to read the numbers)
The speedo is electronics and it gets its reading off the spinning cable by measuring the pulse rate.
The pulse rate value for your speedo is stamped on the back of the speedo housing
There are a few people scattered around the globe who can re-set the calibration to the desired value or it is simply easier to just buy one one those Dakota Digital SGI-5C and manually calibrate the speedo reading against a GPS reading off your iphone.
Otherwise if chasing a replacement used speedo make sure it matches the pulse rate value stamped on the back of your speedo housing