I converted a contractors generator to run on the natural gas that's piped into my house -- it runs smoother, quieter and cleaner. I got the kit from US Carburation.

I have a quick connect fitting so I can plug it into the pipe where my barbeque plugs in. It connects to a manual transfer switch for the 12 most important circuits. Not as nice (or quiet) as an automatic standby generator but it's good for emergencies, and I don't have to go out for fuel or call for a truck to refill it. At least the pipes won't freeze, we can wash cloths and keep the fridge running.