Hi Wildkid, you are not the first one , many are there. I will try to explain you on in descending order.
Option 3 - Complete Big Bold NO
Option 2 - A new CNG kit for car might cost you anything from 20k-30k. Plus added risk of getting caught as it is illegal where you stay. But as you said you are shifting your base, Is CNG allowed at new place? If no then don't go ahead with it.
Option 1 - Yes go ahead and buy Beat diesel base model, let me explain you the calculations.
A-Star sold : 2,30,000
loan repayment : 1,90,000
Money left : 40,000.
You put more :50,000 (as it is you have to put more 30,000 if you install CNG) for down payment and go for Beat Diesel.
EMI of new car for loan of about 4,00,000 would be more or less same as what you are paying now.
I hope this helps.