I would like to know this as well. My bike light is a Vistalite 10W MR11 and I'd LOVE a 150 lumen LED upgrade for it. Any modders out there want to take up the challenge?
Here's the link for Cutter Electronics search engine: MR11 KIT
enter this in the "search for" box: MR11
Scroll down the results page for a stunning array of options. This is really sweet if you can do basic soldering/assembling. Lots of people here can talk you through it.