I found the OP's question interesting, as I have an 83 XR500 with a 6 volt system.
The OP is looking for more brightness by using an LED bulb, but the superbrightleds 6 volt bulb is 19 lumens. A standard incandescent 6 volt bulb is 21 lumens (a standard 12 volt 1157 is 32 lumens).
Things are better in the 12 volt LED area, where 60 to 140 lumen bulbs are available.
It looks like a 6 volt LED bulb will be a bit dimmer than the incandescent, but I think it would be more vibration-proof.