How big is your incubator? If its about the size of a mini fridge I think you'll be fine. You don't want to put a fan that is too powerful and will generate too much heat.

The point of having the fan is try to maintain an even temperature throughout the incubator. A good way to test if you fan is performing adequately is to place thermometers at different levels of the incubator and let it run for a few hours, then check to see how consistent the temps stay throughout.