Why don't they just... (spoilers)
...finish the 1992 film? If they have all the footage, they could ADR the dialogue, redo the score, and foley the sound effects. Finishing the film would have been easier than making a feature length documentary about it.
I tried finding the answer to this and just found a line in an L.A. Times article saying "And while the original "Shirkers" is unlikely to ever be seen as it was intended...". Perhaps the success of the doc will lead to it getting finished.