Can any one explain me that what are the modifications required in engine while using CNG AND LPG as fuel...!?

To do it properly and experience no loss in power or driveability the camshaft should be changed ( valve timing )and the ignition system recurved ( ignition timing ) to allow for the difference in temperature and burn rate of the two fuels. Not a lot of conversion dealers offer this because it is expensive to do when you add on the rest of the conversion cost, but that is how to do it right...