If you do add gas assist, the biggest problem will be drilling into the kettle for the burner tube and 2 screws to mount it. Weber makes the burner tube as a kit. You will need a ignitor button, regulator and hose kit also. There are a few other items as well...so its not so cut and dry. I can get you a full list of what you need if you go that route!
Oh, and BTW...nice score!