You're forgetting an important thing, kero is quite a clean fuel, it's the oil that's the nasty thing in this situation, you also need to think about the properties of methane in terms of flash point heat etc! you could end up cooking a NGV, then you have the fuel parameters in the ECU that have been setup for Kero and not methane. I'm not ruining the idea, it's very clean to use methane but it will need more input than just swapping the fuel etc