I've got the DVD. No extras unfortunately, and it's starting to look rather dated, but the flying's still awesome. The chase through the LA storm drains is amazing.
No mention of who did the mods, but the inlay says that designer Mickey Michaels and electronic special effects expert Peter Albiez, with visual consultant Philip Harrison did the research and decided on the Gazelle. Two were purchased for $190,000 each.
Oh, and if I remember correctly they were also used afterwards in another movie, painted green and minus the mini-gun. Don't remember what it was, but I think it might have been "1984". It was that kind of near-future post-apocalyptic sort of theme anyway...
Si