Since you have a steam system be sure you understand how to properly size a replacement system. Steam systems must be size on the amount of installed radiation, not on the heat loss of the building. If the replacement boiler is to large it will not work well. Seems counter intuitive but that's the case. Here's one reference to start your understanding of the problem. http://www.bellgossett.com/Press/Counte ... 2002-B.pdf
This link is broken, either the page no longer exists or there is some other issue like a typo.
One pipe steam systems are also difficult to balance because so much of the performance is dependent on proper piping installation. If your system worked well with you old boiler great. But if not and you need to make corrections or improvements, I'd suggest buying the book "Lost Art Of Steam Heating" from http://www.heatinghelp.com/
Be sure you understand latent heat concepts and how it's essential to match steam production to radiation. While I'm not an expert I'd guess desirable characteristics of a coal steam boiler would be one that could quickly respond to demand, i.e., make steam quickly or shut down quickly. Boilers with large water capacities when configured for steam use would not be desirable. Take a look at the AHS S130 and get a couple of reference customers using them with steam. Above all don't oversize. Big is not better in this case.