To be quite honest...I haven't read all of the posts in this thread.
To moombadaen...I doubt you'll find the precise information that you'll need for your exact rifle here.
As you can tell...the "rifle" is basically a metal/plastic shell, wrapped around a mechbox and inner barrel.
For the disassembly of the rifle...(someone tossed out a link already I think)...search google. "[as specific as you can get about your particular rifle i.e. Swiss Arms 552] airsoft disassembly" (i.e. G&G MP5 airsoft disassembly....or G&G MP5 airsoft disassembly how-to).
Sometimes you'll have to dig through a couple of sites to find a good pictoral how-to...sometimes you'll find a Youtube video.
Reassembly is most times just the reverse of the disassembly...if you're handy you'll figure it out.
The Sig mechbox is basically the same as a G36 and an AK. Small parts differences here and there, but essentially the same. They're Version 3's. The version 3 is pretty much the same as a Version 2...just a bit of a different shape. Follow any V2/V3 thing on mechbox.com to get the general jist of it (I think the TM M4 change bushings video had a good once-over of the mechbox). Other google/articles on "shimming a mechbox" will help too (there's lots of stuff on this site about it do. "Devgru Shim Guide" might return a writeup with great pics.
If the grip (the one your trigger hand holds) is broken...you'll need to get a new one. It holds the motor in place and if it's all wonky and allows to the motor to slip up and down you'll end up stripping gears. So either bodge it together as best you can...or find a replacement grip. airsoftatlanta, evike, the warstore...dunno many US places.
And....if you find all that a pain in the ass (and it is when you go through all that trouble and it still doesn't work)...just toss it and buy a new one.
These toys are 3-4x more expensive up here and I'm still tempted to toss problematic ones into the garbage sometimes.
Best of luck, happy shooting