Many people make a business of taking stock cars from the assembly line and customizing them.  But one which is factory-modified would be a special edition or one-of-a-kind vehicle built by the original manufacturer, in this case, Honda.  I don't know if the Civic GX is what they're referring to.  I have a Civic Si which has a racing package. You could say it's modified from the standard Civic.  But I believe it's still considered a stock car.  I think the same is true of the GX.  I think they make a lot of them.

I believe your excerpt refers to a GX which has been further modified at the factory to run on LNG.  They may have produced only one of these factory-modified versions, or they may have produced a thousand.

  - A.