PORT OF LONGVIEW, Washington — If Northwest Advanced Bio-Fuels has its way, the Port of Longview may soon have a $2.4 billion sustainable aviation fuel plant. But the mega-project to turn timber waste into jet fuel has faced a slew of challenges on its way to landing at the giant riverfront Barlow Point site, a deal that’s still not inked after nearly four years. The people behind Northwest Advanced Bio-Fuels say the project is mere weeks away from finding the financing needed to lock in a site and build the plant — the first of a handful of additional facilities around the region to fulfill Delta Airlines’ immense need for sustainable aviation fuel. To port officials, however, the project is one of about 20 that have considered its flagship Barlow Point site, any one of which could put money down today and start the long process of realizing a mega-project there tomorrow.