Hope in the Lennox heat exchanger with a CO reading of 455 ppm in the flue PVC pipe

I have an 18-year-old Lennox gas furnace. Recently, both the heater and the AC condenser stopped working, and the diagnostic code indicated an open rollout switch. I reset one of the switches, which temporarily restored operation. However, I decided to shut off the HVAC system after reading online that this issue could be caused by a cracked heat exchanger.

A technician inspected the system and found a hole in the heat exchanger, with a carbon monoxide reading of 455 ppm in the flue PVC pipe. He recommended replacing the entire furnace since it’s nearly 18 years old, quoting approximately $8,000 for a full replacement or about $4,000 to replace only the heat exchanger.

The model has a 20-year warranty on the heat exchanger, so I’m unsure whether it makes more sense to pursue a warranty replacement or replace the entire furnace.

If I go ahead with a full replacement, do you think a heat pump would be the better choice? I’m leaning toward just fixing the heat exchanger if I can get the price under $2,000.