Years ago I supplied natural gas cylinder heads and repair parts the natural gas fields. The mainstay up in Alberta are Cat 3306NG engine. The smallest engines that I seen were the Cat 3304NG. For smaller applications I would look into the 3 cylinder Kubota NG. To bad we couldn't get a conversion kit made for the 2 cylinder Yanmar from the Surplus center. I wonder if Yanmar makes a natural gas cylinder head for that engine.