Now here you could calculate the expense details.
Petrol Engine est cost = 50000
fitment+alteration+oils = 10000
CNG = 35000 (new)
Total Cost = 95000
Less your engine cost = 15000 (est)
Remaining cost = 80000
Now you compare the difference of cost of running on CNG versus Diesel the comparision is as follows.
CNG 450 RS/8.5 kg = 100km (city driving)
cost /KM = 4.5 RS/KM
Diesel Rs 74/liter = 74 Rs per 11 km (city driving)
cost /KM = 7.4 RS/KM
Your savings/km = 7.4-4.5 = 3.1 RS/KM
Your inintail cost for petrol+cng setup = 85000 RS
Now divide it with savings /KM = 85000/3.1 = 27419 KM
In other words after 27419 km of running your intial cost of conversion will become justified. After that you will be getting the benifit of conversion.
Now if your average daily running is 100km then 27419 km = 9 months.
After 9 months if you want to sell your car then your car worth will be less than originaly mantained diesel engine car.
All of your investment will become loss.
Final words. Do cnvert your car if your usage is very very high and only if you dont have any idea of selling your car for atleast 2 years or so.
And always remember to keep your old engine with you so that if ou plan to sell your car for any unforseen situation then you can simply put it back and sell your car as original.