Engine weight doesnt matter that much, you can change front springs only to handle the engine load. Over all it does not differ much in weight in comparison with 1300 cc.
CNG is just headache but its upto you if you can bear with it go ahead otherwise sale it out.
But if you compare 1300 vs 1600 you will come to the conclusion that 1600cc ki average achi hai whereas 1300cc ka kharcha zyada hai.
I am Nissan Lover and I own Nissan 86 and recently I placed 1800 cc Nissan Original Engine in it and throw out CNG. I am getting 12km in local and 15 km on Motorway. CNG one cylinder costs about 750 and gave me 120km whereas now adays I am getting 180km in local 15.50 Litre @ Rs. 1000
So decission is upto you.
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