If the engine was designed to run on CNG then you can be assured of best performance because it was designed to run on CNG, if your are thinking about a conversion I doubt if you can achieved the best performance of cng… Also, don't confuse CNG and propane…not the same thing. A propane conversion is probably more practical than cng. but still why do all that work if you can buy something made to run on propane rather than something homemade.

CNG is a great fuel for spark ignition engines. So is LPG (propane).

Because they enter the engine as a dry gas, they don't contaminate the oil film on the cylinder walls like gasoline/petrol does.

Both gases burn very cleanly.

For these reasons the oil stays looking new for ages - it does not get contaminated by gasoline or burnt products like soot.

Engines running on CNG or LPG can last a lot longer than the same engines running on gasoline.

Both gases can also be used to partly replace diesel fuel! A "thin" mixture of either gas it added to the air steam and it acts as a catalyst when the diesel fuel is injected, making for a very clean burn. In a truck or diesel car, you can replace about 25% of the total fuel used with LPG it CNG. The oil stays cleaner, has a cleaner exhaust, the engine torque is increased, and the engine lasts longer.

The main problem with CNG is that you can't turn it into a liquid at room temperatures. That means that it can only be carried as a high pressure gas, so range it's not as good as having the same sized liquid fuel tank.

LPG can be liquified at room temperatures (like your BBQ cylinders), so range is better when using LPG, but it's not as high as the range on the same volume of gasoline, because it contains less energy per litre/gallon.