I did them. If you do a search on the board you should find some info about the conversions. I believe myself and one other guy here ahve done it. It all has to be made custom unless you use the little replacement bulbs, though I've not heard of anyone doing that yet.
The truth about LED turn signals is that you really need to use high powered LEDs, like those made by Lumileds, a spin-off of Agilent/HP. These LEDs are quite expensive and need to dissipate a fair amount of heat, so heat sinking is important. You also have to devise some control circuitry depending on how you do it. I ended up building a small box to replace my flasher circuit. It had a variable frequency oscillator inside which drove a high current transistor. It was neat to do the strobe light effect
So if you want to do it, prepare to make some circuit boards, and find these pricey LEDs. They can be found online, if you search for Lumileds, or Luxeon emitter or Luxeon Star. There is a slightly cheaper variety that goes by the Pirhana name. The red ones will cost you at least a few bucks a piece, not sure about amber.
There was a surplus electronics store in Northern California that had a superb deal on the pirhana LEDs. I don't know anything more than that, but the point is that you may be able to find a good deal if you hunt around.