A lot of times OLD cast Iron radiators can be salvaged by just removing the leaking section.
I suggested this a few months back with great results IM told.
Depending on where the leaks are what can be used to cure them. For example if only one section is leaking you can have a plumber separate the sections with nothing more then a mallet and a piece of 2 4 .
Once the bad section is removed you can reuse the push nipple to rejoin these sections leaving the bad one out.
If the leaks occurred because of freezing replace the entire radiator
I have welded and brazed literally hundreds of Cast iron and steel radiators with great results as the ones I repaired when I was 17 are still going strong over 30 years of continued service.
If you have to buy new Cast Iron rads try Burnham or Govenilli from your local “plumbing Supply”
If your near Connecticut try home wreckers salvage as they have a fantastic collection of second hand radiators from mansions torn down.
In NYC the Bronx Mikes pipe yard also sells some great embossed steam radiators for about $10 per section.
Get a local plumber to hydrostatically test these heavy things BEFORE installation and prior to painting them.
By the way the ones I did weld 30 years ago were over 80 years old at that time go figure