Boiler replacement Urgency

In general, you don't replace a boiler because of a cracked firebox/combustion chamber liner. You replace the liner. But if the cast iron boiler sections are leaking, then it's time for a new boiler.

What model New Yorker?

Is this a steam or hot-water system?

Where are you located?

As long at the boiler is not leaking it is usually cost effective to repair a combustion chamber. (top red arrow item 21)

I have attached the exploded view of what is inside the metal jacket (covers). if you look at the bottom box of the heater this is made of thick sheet metal but not so thick that the flame from your oil burner wont destroy it. To protect this box from her heat of the flame from the burner there is a ceramic fiber "Combustion Chamber" (item 21 in the illustration) if the crack in the combustion chamber is getting bigger each year, eventually it will become urgent to replace the Combustion Chamber.

as long as the crack is minor the flame will not distroy the Fire Box (item 20)

Depending on the condition of the screws/bolts/studs/or nuts that hold the cover on the combustion chamber, this may be a 1 hour job. If the fasteners are rusted over, then heating them with acetylene torch to free up the corrosion and rust, or worst broken off and need to weld new studs on the firebox, this can be several hours.

Finally if the combustion chamber crack is very bad and the flame has already destroyed the fire box (if the crack is at the lower red arrow in the diagram) then additional steel plate will be needed and additional fire-proof insulation should be added. any signs of overheating on the exterior jacket should be panted over so that new heat damage can be easily identified in the future.

I'll 3rd that..20 years old...in my opinion is nearing the end of it's life. If it's going to fail odds are it will be very cold outside...and possibly inside while you wait for a new one...