Gas with Tankless Replacement

EdB

This summer I plan to replace a 32 yr Utica Starfire with old oil tank.  My single pipe (EDR 800 sq feet) with a tankless water heater serves my needs for 2 seniors.  I was told I will need to purchase a separate gas fired water tank.   Is there a Mega Steam equivalent in Gas?  Burnham will not warranty a gas conversion burner.   What are the advantages to power burners I read on old posts?  Looking forward to hearing from the pro steamers on their recommendations.  Can I assume cast iron boilers are still preferred over Steel?  Thanks for your responses and education.