I did some searching on Google and this is what I turned up as the first result:
It's just a short guide with some basic tips on both active and perventative maintenance of your AEG, though it doesn't really get into the maintenance of the internals. Few results down I found this on Arnies:
That deals with reassembling and cleaning your AEG internals. I only skimmed it quickly, but I imagine that's what you're looking for.
Hopefully that helps somewhat. Did for me, considering I essentially had the same questions as you.