Briggs and Stratton 5hp Horizontal Longevity and Fuel Question

I have a 5hp Briggs that sees about 4-8 hours of use per day under very low load as it replaced a 2hp electric motor and is basically idling on standby until the flow valve opens. I change the oil every month but about how long do these engines last? I got it used about 3 years ago and it has a date code of 2001. Now onto my fuel Question. I have to refuel the engine 1-2 times a day depending on how much it was run. My operating cost is about 1-2 dollars a day in fuel but in the winter months it is a pain to stand outside and pour gas. Are these engines convertible to Natural gas? If so I think that would be easier to maintain for me. Even though sometimes we forget it is running because we cannot hear it.(golf cart muffler welded on) If there is such a kit to run on nat gas I would appreciate a pointer in the right direction. Thank you all for your time! :)