Missing Hot Water Radiators: Options?

Hi all;

I have a 3500 Sq. Ft. (above ground) three story house built in 1917 in Detroit, Michigan. The original coal fired gravity hot water radiator system was converted to a circulating gas system, with an American Standard 300,000/240.000BTU gross/net boiler (Model 1B J1 - G405); my guess this was done in the 1940's.

Sometime later, radiators in three rooms were removed for un-known reasons. Two of the rooms are "sun-room" type with  lots of windows; the take off connections at the mains are still there, but the piping is gone. The third room is a basement "speakeasy" rec room (Michigan got Prohibition in 1916), that had ceiling mounted radiators; the take offs are still there.

What can I use to get heat back into these rooms, they are frosty and affect the adjacent rooms. The house is a renovation project, so ceiling  access from below is not so bad. Used radiators are getting somewhat scarce, since we have a big scrap theft problem here(scrap companies are unregulated here). Baseboard fin tube? Floor radiant (however, I like carpeting)? I am scratching my head on what to do in the basement room. Anything else? Can I use copper pipe (or?) for the supply pipes?

Thanks for your help.

Mark