You will need to replace the entire gauge. Mercedes speedometers are matched to the differential, tire diameter and engine.
The speedometer has a K value (W= on W115/114) stamped or printed on the back - if it isn't rubbed off. The K value indicates the number of cable revolutions per mile (or km). The odometer gears in the speedometer head are selected for this value. The speedometer itself is calibrated to this value.
Furthermore, the shift markings on the speedometer face must match the engine. Diesel engines will have a lower redline and earlier shift points than gas engines. As an example a 74 240D and a 72 250 both share a 3:69 differential and both speedometers will indicate the correct speed in either car but the shift markings will be incorrect for the engine type.
A friend once brought over a 240D with an 85 mph speedometer. Everything looked normal but the indicated speed was off slightly. What caught my eye were the shift points which were much higher. Turns out the speedometer was from a gas powered car with a 3:58 differential. The difference between 3:58 and the 3:69 in the car wasn't huge so nothing seemed amiss but when I rode with him and he ran it through the gears I thought the engine was going to explode!
If you aren't sure if the one you have is correct you can determine the K value of your speedometer by counting the odometer teeth and determining the ratios in the odometer gear train. Then calculate the number of cable rotations necessary to cover one mile. The number will be somewhere between 1200 and 1500.
The K value needed for your car can be determined by calculating the number of driveshaft revolutions per mile and dividing that number by the speedometer drive gear ratio (2.166). Use an online calculator to determine the circumference of your tire.
The K 1425 value shown in the picture is for a W123 240D with a 3:69 ratio. The calculated K value for the car is 1420 - close enough. Interestingly enough the K value for a W115 240D with a 3:69 ratio is 1390 due to the higher aspect ratio of the earlier car's tires. The calculated K value for that car is 1388. - also close enough.
In short Mercedes speedometers are application specific and any mix and match is fraught with peril.