I20 Magna Petrol 2013 model CNG fitted from outside

Hello team,
I bought a preowned Hyundai i20 magna 2013 petrol model in dec’17. Previous owner had installed CNG kit in it from outside.
Problem1: Coolant is coming out from silencer when i am starting the car. Hyundai service station says, "we need to open the engine to check the issue. if air gas kit it broke down, 40k - 50k cost.
What should I do ?

Problem 2: Should I remove CNG kit, would it improve average. currently I am getting only 8 KMPL.

Pls help. Saurabh