We had the same problem with S110 Benly 1980(Japanese), one-unit headlight and speedometer. Had it changed with separate headlight and speedometer. It was much cheaper than the original one-unit(new) and about the same cost as a used one-unit piece. That was over 20 years ago, I doubt you will get a working piece now.
If you must keep the original housing, take it to Karachi and have an expert replace the speedometer mechanism, it might need internal modification to mount the new mechanism. Ask Karachi members for expert speedometer technicians.
If keeping the original is not a must then go for separate units or for temporary use. You should be able to get it done in Nawab Shah as well. You will require extra bracket for the speedometer.