There are several kits available.
I got to looking into this a while back. Most kits will run propane, natural gas and gasoline with a quick change-out.
A couple links w/ info;
This one is loaded with info on their product.
Generator Conversion Kits to Propane and Natural Gas.
I know a local guy running a an old e150 van on propane w/the I6 300 engine.I think he called it a Nash kit. Says it's trouble free.
Take this as a starting point for research not an endorsement, as I have not ordered a kit, yet.