I've been pretty big into gameboys the last few years, especially modding and backlighting them. I'm also a big pokemon fan. When I found out about the Pokemon Mini, it wasn't more than a couple weeks until I had one in the mail.
I also found out you could backlight these things, but I didn't have any old Siemens phones laying around. I did however have a partly damaged gameboy backlight laying around. I went ahead an cut it to size and installed it in my Mini.
It works great! You can see pictures here > http://imgur.com/a/5jjp2
I basically just cut it to size and installed it the exact same as the siemens backlight. It even has the diffusion layer built in.
I got the backlight from "handheldlegend", a gameboy modding parts site that has a ton of backlight color options. I just happened to have green on hand, but they have most any color you could want.
I would like to mention that I used an older and thicker version of their backlight that has been discontinued. This older version gives some pressure hot spots on the screen, as seen in the photos. I would imagine their newer thinner backlights would not have this issue at all.
Let me know if you have any questions!