I can tell you the Local Gas Company ( now...Enbridge? was Dominion, and West Ohio Gas in the before times) has all their vehicles running on CNG...
but the LPG (Liquid/Liquefied Propane Gas) conversions that were fairly common in Europe/Australia, were never really approved for sale in the US.
and Most of the CNG (Compressed Natural Gas) cars/ trucks we've seen in the US, have come from the factory that way... there probably were some conversion kits available in the 90's, but I've not seen or heard of one in years.