@OP You can swap 1.6L engine in your car i.e SE Saloon's engine. Install it and then install CNG. Take the engine recipt on which engine no is written to the excise and taxation dept and they will add to ur book. I also have 2.OD saloon. A showroom guy told me abt this and said "aap apni gari SE Saloon kr k bechna". I was lyk WTF :S
Another method is to install CNG on your current engine. After 100km/h your car will automatically convert to Diesel else it will run on CNG.but as said by ILLdude Gari ki Waj jati hai