Yes some of those items have already been replaced. The last replacement was the fan belt. I've been told the crank is making sounds but if oil isn't being reduced (which it isn't) then I can continue driving.
I got some CNG tuning done too. Previously the CNG tank hardly made around 90Km, but after tuning it is performing better. Our latest tank recently did around 145Km in mixed city and highway driving. Haven't accurately measured petrol yet (filling full tank is a bit expensive :)). But the mechanic also showed me the spark plugs and they were black. He said this indicates high petrol consumption.
Any idea if I can have Enercon check engine tuning for second opinion? I'd like to have at least 12Km/Liter in City driving and 14Km/Liter highway.