Every incubator I have used or designed has had provision for gentle air circulation. In one case, we used the equivalent of a modern PC fan in a box attached to the side of the incubator box. A incandescent light bulb in the box with a thermostat switch kept thinks at constant temperature. The hard part today might be getting the incandescent light bulb. Any heater, such as mentioned, will work.
Tony didn't mention it, but you also need some way to rock the eggs, as shown in the picture he posted.
John