Yes, Cast Iron radiators can be repaired, depending where the split is located.
The crack is normally stopped by drilling a small hole at the end of each side of the crack to prevent this crack from getting bigger during the welding/brazing process. The Cast Iron must be slowly heated and VERY Slowly cooled to prevent more cracking.
Then using either a cast Iron welding rod Or bronze filler (brazing)the crack can be filled.
There is something called Black and White which is a sulfur stick that is can give great results DEPENDING on temperature and pressure.
What I normally do is remove the bad section if I cant get a good weld position and just remove the one or two bad sections.
Contact either a Licensed master plumber or a good welder from a boiler company.