COSTUME: Lots (see below)
A custom of many spare parts...
Mr Fantastic's boots
Iron Man's yellow lycra costume
Storm's shorts
Doctor Doom's holster (or at least a plaster cast of it)
Falcon's white PVC cuffs (painted black)
and, the knob-tops off Falcon's wing harness for his ear pieces!
Milliputted on a cowl and his eyes (of course, every picture I've seen since hs YJ with 'goggles' rather than separate lenses. D'oh!) and used a couple of glass-headed pins for his antennae (not true to *most* renditions of YJ, but they look good to me).
Then it was just a case of adding a black lycra chest/back strip (and collar), a pair of craft foam wings, a belt from the same stuff, his insignia (painted on) and another Avenger was assembled!