Natural gas conversions have been available in Europe for decades, for pretty much any automobile. Logically, it should have Long ago gained traction in the US with our vast NG reserves, but it hasn't. The reason? I blame the ridiculously low gasoline tax compared to the rest of the world.
Want mass adoption of alternative cars and fuels? Just raise the gasoline price to $7-8 per gallon, sit back, and watch it happen.