MR11 probably pretty easy... SSCP7 with Fraen reflector or Cree MCE with Fraen reflector. May need an aluminum spacer to mount the LED on and battery pack of the proper voltage.
Hey Aqua-Andy,
Were you able to make this conversion? Did you have to do anything about voltage? I also have a NiteRider light and would also like to make a conversion to LED. I'm not sure which model bulb that I have. It says #2, 10W, 2640 on the lamp housing.