Get a second opinion from a trustworthy contractor. Some utility companies do free safety inspections on gas appliances. Have them document exactly where the crack is located. An honest tradesman will have no problem with that. A dishonest one will refuse. My typical wording goes something like "HX cracked 2nd chamber from right, right side, 3 inches back, 2 inches up". Such specificity leaves a technician wide open to being refuted if he's incorrect.
Post the model and serial number and someone will verify that the heat exchanger is or is not available and its warranty status. Some heat exchangers come with a lifetime warranty. Many come with a 20 year warranty, which means you pay just for labor and warranty processing. In some cases an OEM will credit you the cost of the HX towards a new furnace if the old HX is not available but still under warranty.
If it is actually cracked, you may be better off with a new unit even if a heat exchanger is available. However, that's for you to decide. Do your research (as you are now) and make an informed decision. Never take any one person’s word for it unless you know them inside and out.