Want to replace radiators. Prewar Boiler ...Need advice

I was told by national grid I have a prewar boiler and would last forever. Bought a row house 3 years ago and so far no problems. I know its a hot water system but I was curious what the black water tank hanging over the boiler is used for.. see attached photos. Does this mean its a gravity system? I’m looking to replace the radiators to slimmer units. Can I use hot water hydronic radiators? Any old timers out there that can give me advice on this beast? The plate on the boiler says National – US gas boiler. No 7-22\. Steam 330 sq.ft, Water 92000 BTU/HR, Input 153000 BTU/HR, Output 122400 BTU/HR Thank you