I remember someone was making reproduction thumb buttons, i think they were selling them on ebay. Try searching the forums here, there was a link, but it might have been a year or two back...
Someone should just start molding the thumb and trigger buttons, they're commonly missing, and they'd be simple to produce. If you're missing the microswitches, they're commonly available at Radio Shack or Fry's, or you can order real cherry replacement switches from Happ.
Looks like you got a decent deal though, for what IS there!