Good and bad news. I have the IC2 Classic source. Also running smoothly (still need to fix decompiling bugs) and i know that i can make most mods compatible to it (as long they only use the API and not to much inner classes). So Gregtech/(Maybe Nuclear Controll would not work with it).
Now the bad news. As immibis said even if i get the source (not from him) i have no rights to share it so sadly it will be only in my private use. And also i make a safty code in that only a few people can play with it (private mod simply) when i get the rights from the IC2 team to release the source (only 1.6.4 someone else can update it. I will not support 1.7.10 at all. Sorry) and the mod then i will do it.
Do not hope on that one. Because IC2 Classic from immibis is allowed because it does not support mods so it can not replace ic2 Exp.
My ic2 Classic version (which is named IC2) is able to replace the original. And thats a thing what the IC2 Team will not let it happen.
But i can prepair support to special mods that would normally crash with that thing. So if i get the rights when minecraft is at 5.0 or so many yell at the ic2 team that they give me the permission then i will release it.
To the current state: I managed to get the source running (do not know how stable it is. Stable enough i would say).
I am still backuping the stats i made and prepair to install the new API (1.6.4 of IC2 Exp) to this mod.
I made yesterday a try to install it but forgot to make an backup of the stablest source i had. So i restarted (it was only waste of 6 hours not much).
Also a note i will show what i did and what not in my world roundtour/modupdates to the ic2classic mod because i am showing my private work this would not go against the licence of ic2 as far as i know. If i am wrong correct me please.