"Tutorial" on how to mod a 25 years old mobile phone to work as a HTPC

Oh, and the reason why I wanted to do this. My old and trusty Xbox1 with XBMC has been streaming all our videos for us but the only problem is that it isn't powerful enought to decode these new high definition file formats. I
happened to have a 3.5" form factor motherboard and as the new XBMC builds for Windows seemed stable enough, I decided it is time to update the box and build it inside a 25 years old mobile phone!