The only people who don't recommend CNG are those who have never owned or have experience with a vehicle with a CNG system or get cringe whenever they hear CNG. Yes, you'll have to sacrifice the vehicle's power a bit, and the maintenance will be a little more, but it never reduces engine life. We used to own i-DSI City and EFi Cultus for five years and never had any issues, especially related to the engine. The only issue we had was both cars got stranded. I was in early-teen back then and I believe it was due to a fuel filter getting choked, probably because of how bad quality or outdated petrol used to be before it upgraded to Euro-II 2012.
The only bad experience I had with CNG was with any vehicles that have below 1000cc engines. Those cars are already not powerful enough, yet already have good enough fuel economy. With the CNG system, they are almost undrivable. We used to own an automatic Cuore, and we ended up selling it nine months later. It was decent in petrol but was a disaster in CNG. With AC on, it was almost undrivable, we have to press an almost full pedal just to get it running, but the engine had to run really hard, ruining the fuel economy. I believe this is the real reason why engine life gets reduced, especially on these really small cars. I remember one time the car just gave up on an inclined road, and it was dangerous. I really wished CNG was never allowed in vehicles having below 1000c engines, and CNG might still be commonly available.
The 1300cc Corolla automatic is already so underpowered plus horrible in fuel economy, with CNG it's gonna be almost undrivable and you're going to hate the car even more. Unless you're going to do an engine swap, it's better to sell it, spend the extra cash, and get a 1.6 Altis, you won't be disappointed. Just follow the maintenance based on the owner's manual, and this will guarantee better fuel economy.
