wow,you want to cruise at 40mph/80kph on a bicycle? i hope you have a good bike.
helmet and full body suit would be nice.
my 48cc with 44t sprocket goes up to 32mph,but i cruise at 20-25mph
you can go up to 35-40+mph with a smaller rear sprocket(32-34t) or you could get a jack shaft kit which uses the bikes gears(but it cost a lot for the jack shaft option)
go with a 26" bike, a 48cc 2-stroke, 32or34t sprocket,performance muffler,upgrade sparkplug and wire,you might get 80kph with this setup.
and you can order online and get it delivered:
zoombicycles.com $109.95 free shipping