DON’T use copper baseboard units unless you are converting the entire house or you’ll be one unhappy fellow. They have totally different heating characteristics.
While new radiators are very expensive, you can often find good used ones at salvage yards for next to nothing.
You can also disassemble and repair the cracked ones, depending on how many sections are cracked. If the section sizes are the same, you might be able to turn 6 cracked radiators into 4 or 5 good ones.
Good luck, Chuck.