We consult on and install many steam to hot water boiler jobs here in St.Paul/Minneapolis and around the country. 100 years in more the average in our neighborhood, than a number to worry about.
First, the cost of available fuel in your area.
Second, measure the radiation to see if there is enough to replace the heat lost using a lower operating temperature than steam.
Third, forget about low-efficiency cast iron boilers.
A condensing boiler will thrive on the average water temperatures needed by most steam to hot water steam conversion systems.
There can be quite a bit of re-piping to do, experienced experts need only apply, but the outcome will be an improvement in comfort and likely a very considerable fuel cost savings.
Talk only to those who are installing condensing boilers. They will likely also install steam, and low efficiency boilers but not the other way around. These are the smart guys.