I need to replace the motor in my attic vent fan. The only specs given on the old motor are 120V, 60 Hz, 2.6 amp. The only motor I found at Home Depot is a 120V, 60Hz, 1.9 amp with an external capacitor. Is this a suitable replacement? What are the critical specs when replacing an electric motor?

The motor at HD is probably not going to do it for you. The original motor was more powerful and the manufacturer installed a blade to match. I would suggest going on the Internet and researching additional motor options available. Emerson has a large line of fan motors. This might help your search Furnace Fan and Blower motor repair and replacement.