The engine number is mentioned on your registration certificate and user's manual. On the car, you can find it on the engine itself and vehicle information badge right next to passenger side door slit and on the car's floor on driver's side.
You want to service and repair an old Tata Sumo, then you have to contact your nearest Tata service station. You have not mentioned you city. So to check the details of the service station according to your city, please visit the link below-
Thanks for your suggestions Mr, Partha, these will help us for sure. However, BSIII stands for Bharat Stage 3, which is the emission norms, while the CS, LX, etc. are the monikers to define different variants.