You CAN probbaly stick an LED bulb in there. Whether that "bulb" will provide the right illumination to make your entire lamp still conform to COLREGS is something else again. There are a number of long threads about that.
I have replaced incandescent bulbs in Aqua Signal series 20 and 40 lights with aftermarket bulbs by Dr. LED with satisfactory results. The combination does not carry USCG approval. Dave