I got a Tri-Fuel kit for my gasoline Honda genny from Century Fuel Products. From my research, you can convert a gasoline generator with a tri-fuel kit to run on gasoline, natural gas and propane. I have been looking at many different companies that sell these conversion kits, but I went with...
I googled "motor snorkel" and they have a cool idea. But I do not think it is safe, because their venturi insert is made out of plastic and rubber. :( So I went with Century Fuel Products' kit.
Here is their link: Generator Conversion kits to Natural Gas or Propane Page.