[broadgage]

Many older electric fires incorporate a fake fuel effect, intended to resemble burning logs or coal.

Often a couple of transparent amber "fireglow" lamps, the rising currents of warm air from these causes a spinner to rotate under the GRP moulding and gives the illusion of burning logs or coal.

Best IMHO to keep these in original condition if possible. If however the spinners have been lost or broken, then an alternative "self flickering" light source is worth considering.
Several suppliers on ebay offer "LED flicker flame bulbs" these contain numerous small orange LEDs connected to a pseudo random flicker circuit.

They give a reasonable effect without any moving parts. They use little energy and seem long lasting.
Various colours are available, the orange or amber is the most realistic. A very large fuel effect fire that uses several lamps may benefit from several orange lamps and one blue one, thus partially simulating the blue flames and orange glowing embers that occur with some real fires when burning smokeless coal.

The green lamps dont simulate any likely real fire but produce an interesting effect that children find entertaining.

Also applicable to the more elaborate fuel effect fires with a now non functioning motorised fuel effect.

And of course to gas heaters that include a "real fire" flicker effect. Any modification to the gas burning parts is foolhardy, but fitting different electric lamps seems OK.