When the Keymates first came out, CPF member lambda modified the light to a higher output LED utilitizing the N cell...however that was a number of years ago. (IIRC, lambda was one of the earliest, most prolific and talented flashlight modders on CPF...in my opinion.)
I just checked, and lambda hasn't logged onto CPF since January 2007. If you pull up his name in the Members List, there is a link to his website. I don't know if the website is active.
What I found interesting about his website, is a link to another discussion board, Custom Light Factory. Perhaps that board has some information regarding modifying the Keymate.
I also just searched CPF for "modified keymate," and there were a number of different threads that seem to address upgrades to the light. Looks like some good info out there, and a good place to start.