Well, I gotta warn ya that you've set yer sights a lil high lol - there's no "kit" engine that'll getcha 40+ particularly if yer a "big guy" - pretty much all the kits, 2 and 4 stroke will do about thirty out of the box... some less. Unfortunately there's no american made kits either.
however the Morini which is a quality engine with pretty much the highest power to weight of the engines available - you can check 'em out and the bikes ppl have built here: Morini Motors
BUT they are not
a kit and can be a challenge. If this is your first build you may wish to consider an inexpensive Chinese in-frame kit to learn with.
Later when you've sorted it out you can upgrade to a Morini using the accessories from the Chinese kit (throttle, tank, sprockets, etc)... Just a thought