If the rights can be had, I might suggest the title Fast and Furious: Vanishing Point
You would have flashbacks to whatever the plot was in the unfinished film, with exposition. The film would end rather like Vanishing Point, with Walker being made a mythic hero.
This has been done before, with the movie Targets: http://en.wikipedia.org/wiki/Targets
Footage was used from another film to great effect.