Gary,
There are a lot of US companies doing Propane/LPG conversions, and I'm sure when you start digging there will be dual fuel, switchable systems, too. In the midwest we've had propane run trucks for years - I remember seeing cars and trucks that had propane tanks back in the 50's. Here's the first listing when googling Propane Conversion Kits, Propane Conversion Kits – Convert Car to Propane – LPG Vehicle Conversion
and there are selections there for kits, DIY kits, and companies that do the deal.