EFI engines can run on CNG without major changes. The big expense is for high-pressure tanks, regulators and fittings, since CNG is dispensed at up to 3600psi. This differs drastically from LPG systems.

My brother has owned a CNG (factory) Crown Vic. Now he has a CNG Honda Civic. Both engines looked no different than the gasoline versions. CNG runs so clean that the tailpipe has no soot at all. It looks like the engine was never started.

Jim