I converted my 11 HP Generac portable 8 years ago to a tri fuel unit using a kit from this company:
Generator Conversion Kits to Propane and Natural Gas.
Best move I ever made, I did have to upsize my existing NG piping due to the location that my generator would be placed in the event that it was needed. As for the company great technical support... If I recall correctly there is approximately a 10% power loss because of the btu's of natural gas...