A hack saw or electric reciprocating with the proper blade should be able to cut through them without trouble. But are you sure they are not hollow? That is if you cut the legs will you put a hole in the radiator (very bad)?
What kind of plumbing goes to the radiator? Can you remove the riser and replace it with a slightly longer one -- even if you could cut the legs without a problem it's probably easier to replace the riser. If it's old it could be rusted and difficult to remove but that's where I'd start. How difficult was it to remove the radiators in the first place?