For how much? Which brand?
Its easy to install mechanical pump. It takes the mechanical feed from the cam. There are just two nuts. Try to use a new gasket, its hardly for 10 Rs. The pump has one input and two outputs for petrol.
The main bigger output goes towards carburetor while the small output is the return line which goes towards tank.
As you were using electrical pump before that so you will have to remove that setup first. Also, now you have to use the solenoid. Its a valve which stops petrol from reaching the carburetor when the car is on CNG.
There are two configurations for that.
1) To instal the solenoid at pump input. This will keep the pump dry when the car is on CNG and the dry diaphragm leaks.
2) To install the solenoid at pump output (the one which is going towards carb). In this way, the pump will keep working even if the car is on CNG. Some people say that this configuration also has a disadvantage that as the return line is very thin, a constant pressure develops at the diaphragm and it leaks.
I have used the configuration no 2.
Rest @xulfiqar and @smijaz can guide you better.