Don't bother trying to generate 120v from it. Pick up a GM alternator from a local parts store or junk yead and use the Engine and alternator combo to charge your low voltage battery for your backup system.
Its pretty easy to drop new wires though the walls, espically low voltage 12v wires, crawl in the attic and find the top of a wall, drill a hole and poke a stiff wire in then cut a hole near the bottom for a box and usually you can reach in the hole and grab the wire you have fed in. I use an old 8' cb antenna since it will bend enough to go through the hole but spring back straight inside the wall. You do the same thing to run phone or network wires.