They do make replacement BULBS, but I don't think you get the light output of an incandescent bulb. Led's are directional and are hard to to see from any angle other than straight on especially in the bright daylight. They are great at night although.
Believe me, I love led's but have not changed any front or rear lights on the wing for that reason.
I replaced the OEM lighting on my 2008 Red GL1800 to help solve a visibility problem... the red light/brake assemblies on the trunk and sidebags of a red bike are hard to see. I switched them for Show Chrome units, and the difference is impressive... like staring into the heart of the sun with the LEDs. I have noted that some the the LEDs are aimed a bit different to make certain that whoever is behind you is going to see the brakes and turn signal. They are a bit pricey... but so is a rear end collision.