Something is very VERY wrong if your radiators cracked from "too much steam" as

1- These devices can hold much more than the 15 PSI steam pressure and Once one cracked the pressure should have dropped right away.

2- Your boiler should be equipped with a 15# Safety (POP) valve that should have discharged the excessive pressure. (15+ is excessive)

What could have caused these two radiators to crack is water hammer ESPECIALLY with the velocity steam is capable of achieving.

Now repairing a Cast Iron radiator is not that difficult if you're lucky where the rupture occurred. If you look at your steam radiator you will see rods holding these sections together if you loosen these rods you can remove the damaged section and reuse the steel push nipples.

Another option is getting a great Sulfur product called "Black and White" you apply this to the cast iron crack with the use of a blow torch just heating the base metal AFTER really cleaning the heck out of it. It will hold for many years BUT you must use it out doors if you have a large crack to fill.

When I repair a cracked boiler in the winter and I don't have the time to replace it I just drill a hole on either side of the crack to prevent this crack from expanding and I weld this cast iron. But the key to welding CI is the cooling down period which must be slow as not to thermo shock the weld/base metal.

If you contact "Mikes Pipe Yard" on Boston Road in the Bronx section of NY they can sell you 2nd hand radiators about $10.per section. The shipping may not be worth it as you have no idea if these 2nd hand ones will leak or not.

Personally I would opt for removing the defective sections if possible OTHERWISE just replace these radiators with new ones.

If you plan on using your existing radiators give them a hydrostatic pressure test of 1 and a 1/2 times your normal working pressure.

You can also braze these cracks with a Bronze filler rod which is not that hard to do BUT get a good Quality rod like Union Carbide 25M. The main thing right now is to find out what exactly caused these 2 radiators to crack before you do anything else.

I work on buildings with the original steam radiators installed in the late 1800s and they are still in pristine condition.