Phone with Intel inside... it could be intresting... may be phone will be much faster than in ARM processor, but i don't think it will be saves more battery than ARM... we will see :)
Stock ROM u can find here:
http://www.sammobile.com/firmware/
Register there, then write in search box "i9100", choose ROM for your region/country, download and flash into phone with ODIN.
FAQ how to do this u can find in SGS2 section on this forum.
Greetings
Everything about this u can find here:
http://forum.xda-developers.com/showthread.php?t=1457458
Do all steps from there and your phone will alive again :)