I would caution against using LEDs for fire. While they avoid the problem of getting hot (if wired correctly), they tend to give off an unnatural, constant glow. I would try Night Owl's solution first, and as backup try the following: take a flashlight or other incandescent light source, place it below your set pointing upward through the fire, and then intermittently use a piece of tissue paper or wax paper between the light and the set to create a slight degree of flicker. You don't want a lot, just enough to establish that there is a light source with a constant glow, yet changing intensity. Another option, that may or may not look better, is to install a dimmer box connected to your light source; you could just use that to adjust the amount/degree of flicker.