What to look for in a new water radiator?
There are a number of companies that make very low profile radiators such as this one:



There are several factors such as the type of system - steam, hot water; if steam, whether one pipe or two.  You will also have to accomodate certain valves, so placement is an issue - piping through the wall or from the floor, whether you can accomodate valves in the wall, and how to access if you do.

Size is also a significant consideration and you need to determine how much area you need to replace a much more massive old radiator.  While the new ones may be more efficient, you may need more surface area, depending on the overall demand for heat.