You can disconnect the old radiator at the union connection which may be integral to the valve to the specific radiator. Close the valve and you can keep using the system. Then you need to calculate the Equivalent Direct Radiation of the old radiator. That calculation can be done using this - * dot colonialsupply dot com/resources/radiator2.htm
Then find the new radiator with the same Equivalent Direct Radiation as the old one. I am assuming you are comfortable with the amount of heat you currently have in the room. Then install the new radiator. If it's a 1 pipe steam system, make sure there is a very slight pitch toward the end of the radiator where the valve is. I assume the new radiator comes with an air vent. If not you may be able to reuse the old vent. Be sure you choose a radiator designed for steam. The air vent tap will be about 2/3 of the way from the floor to the top of the radiator.