Chap mei makes a bastardized CH-54 in both their soldier force and dino valley lines....i think this has been mentioned in the past. Soldier force is virtually impossible to find but dino valley is fairly easy.
I scaled out the cabin and engines from some pictures and they are about 1/20.7 scale, which i figured was close enough. So when i get it i'm going to build a new 'fuselage' (which is basically just a long girder beam) and graft the cabin, engines, tail and an enlarged undercarriage onto it. I figure it will give me about a 41 inch long Tarhe to play with (should be 46.7" in exact 1/18 scale). I think i will sling load one of my 21c Hueys under it.
No sky lifters on ebay but plenty on amazon. Just search for 'dino valley sky lifter'.
Here's a gi joe forum conversion thread which shows lots of pics of the basic helicopter. Doesn't look like much out of the box but the potential is there...
http://www.joecustoms.com/forums/viewto ... =14&t=1074