What options exist for replacing or upgrading heater radiators?
Cleaning the existing radiator is a good idea.  Now, before replacing with baseboard units, a couple things to consider.  You want roughly the same heat output rating (BTU/hr or similar), which may be difficult to calculate for the existing piping.  (Any new basebord finstock will have its rating on the packaging). Most likely you'll need a long-ish baseboard run to match the existing radiator's output.
Unless you have cosmetic objections to the existing radiators -- and keep in mind you can probably buy different covers, or re-paint the ones you have -- you might as well stick with what you have rather than rerouting the feed&amp;return pipes to fit a baseboard run.
BTW,  if the wall behind the radiator is not insulated and preferable covered with reflective metal foil (to maximize directing heat into the room), you should do that.
edit
I looked up a few vendors of replacement covers. You will have to check the desired dimensions, of course.
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