Asim Sb.,
I have no doubt that your W109 is a very special car indeed. From a performance point of view it is far superior to the original 6.3. I hope you will consider what I am about to say next. You can build a tachometer that can look "almost" identical to the original one from a 6.3. I have done almost the same job on my Mini 1275GT (which had a dead tacho) and to the dead water temperature gauge on the Jaguar.
Recreating the face of the gauge is the easiest part. That is just printing a high def picture of the face of the gauge on photographic paper but it must be printed to actual size and cut round. Then you take the clock fitted to your car and remove the clock needles and internals and keep them safe for the next part of the project. As I remember the Merc gauges do not have opaque numbers that are back lit like the Japanese cars this is why the picture idea will look the part. The tacho photograph is then pasted on the clock face. Next you have to get a tacho that is compatible with 1UZ ECU and transfer the internals of the Lexus tacho into the W109 clock housing. Next the needle is cut to size because the W109 tacho is smaller compared to the Lexus tacho. I think you get the idea where I am going with this. The most expensive part will be the Lexus tacho. Of course before the conversion is done, it must be confirmed that the Lexus tacho internals can be accommodated inside the tiny W109 clock housing.
Regarding the clock, that is a bit difficult since clock housing is not at hand. Again the clock face is created in the same way as the tacho face - picture print. For the housing, a suitable small rectangular box needs to be found deep enough to accommodate the clock mechanism. If not, then the silencer walla can fix you up with a fabricated box. A square face bezel needs to be fabricated out of brass, machined and chromed. Brass is selected due to the good quality plating you will get plus it will not rust ever. The clock mechanism removed earlier is fitted with the same needles and fitted into the place you have selected in the dash. A method will have to be devised to illuminate the clock to look like the original.
In this way with a bit of out of the box thinking your car can have the desired tacho at a reasonable cost.
