replacing one or two of six radiators in one unit of a 90-unit building circa 1925 with steam heat.

I have a classic six in a 1925 co-op in NYC. I now have six active original radiators and want to replace at least one or two of them with thinner units that will hug the plaster/masonry/brick wall and be thinner to give me floor space. Shape flexible, but it must use existing steam in/out connection. I'd like to have better valve control so a really efficient and long-lasting heat sink is good. To optimize function retention and am willing, e.g., to add a heat sink, use iron replacement units, or make the units longer so the central heating isn't defeated; maybe a locally controlled boost? I saw one unit that goes up the wall. Your valuable advice is requested.