OK... suppose I'm kinda a newb on this forum, but I've been building, racing & riding motorcycles and cars for a few decades now.
That government study? It's full of holes and gross assumptions. It's also partially backed by the same funds that funnel to Green Peace, so just might be a bit biased. :)
I have put my old KDX 200, RD 350 and CR 500 on a 4 gas emissions analyzer while on a dyno... tuned well (and that's the key here) they are not nearly as bad as that study would have you believe. Trees aren't going to die as you ride past and the pickup you hauled the bike to the trail really probably did dump more hydrocarbons into the atmosphere.
Really want to be green? Keep an old bike in good running condition and enjoy it... that's using far less resources than buying a new bike.