Unless you can try it well before hand, I would not count on a portable generator working, even if you could get it hoisted to roof level.
You may be able to find a surplus industrial-equipment dealer that would rent a 3-5kVA 240VX480-120/240V dry-type transformer, to set up in a 480-120V configuration. With this, you wiil also need a [surplus?] 30A 600V fusible disconnect switch to connect to the primary of the transformer.
Good luck on your project. A long extension cord from a 120V outlet may prove disappointing from a voltage-drop standpoint. [The transformer may weigh as much as or more than the welder, but may be the only way out.]
