You may have to go with forced air if it is cheaper. Only way to come out on top, is if the walls are all open to be able to pull new piping, whether it is PEX, Copper, etc., along with installing a new more efficient boiler. I am going to guess around $10-20,000 for parts and labor roughly.
A month and a whole lot of work later, we're about fixed and finished now. Two elbows were blown out going to one upstairs radiator. Plumbers went up into the ceiling and fixed. Same deal in the living room, both 90 degree elbows blown out. Fixed those. All other piping OK in this 2144-square foot house. All downstairs radiators done for. Huge living room radiator and pretty big dining room radiator shot. They were 1700 pounds in total of scrap. Smaller rad in pantry shot. Teeny tiny bathroom rad shot. Pulled them all out and capped to the the system tested. All pipes that were repaired held, and so did the others. By some miracle, as we pumped to the upstairs, all four upstairs radiators survived this several week freeze up, as did the boiler. Water line coming into the house froze and burst at the gauge, but by some miracle the boiler was OK. Had the guys do some work on the boiler while they were at it. new expansion tank, replaced absolutely old and nasty parts of various sorts in the piping. Put in a new gauge, the old one was completely unreadable. Cleaned the burners.
This company, god bless them, in our city are real pros at this stuff, we had to go through a couple of heating/plumbing co's before we felt like we had someone who knew hot water radiant heat. They also had radiators in storage, old ones that work, and they managed to put a couple together for the living room and had one for the dining room, same with the pantry. All the same size/shape/btu etc. that was a miracle, too. Got those loaded in and installed. All new valves put on all the radiators. One more rad to put in, the small one , this week. But we're pretty much done.
Heat is on, house feels good, warm and toasty, even, balanced, nice!
So THAT is the update. We're broke now, but boy the house is cozy!