In all my conversions, I always replace any bulbs with LEDs. Generally, decoders have the lighting outputs as 12v sinks (negative). This presents some problems in using LEDs as you have to drop the voltage and limit the current with a resistor. The CL2n3-g is a handy little device the size of a transistor that does this for you. No calculations or resistor values to worry with, just hook up your LEDs to one of these. This also allows you to wire several LEDs in series so running lights or number boards are easy to hook up. These can be ordered from places like digikey.com or mouser. I buy them in bags of 100 since I also use 12v out on my layout for building lighting.