Gotta Need 4 Speed

Is it possible to build a gas bicycle with maybe a 50cc or a 80cc engine (safely!) with out falling to pieces that maybe could achieve speeds of maybe 30 to 40 mph.
keep in mind with these speeds ive listed im not going to throttle it all the time so you cant and you dont say its overkill, how could i make a contraption like this is it possible? would I need a larger engine? i guess its like a whizzer bike.





(Gotta Need 4 Speed)

A couple of other things 2 say:
I while ago I was going to make a 6.5hp go kart capable of reaching skin tearing speeds of 40 maybe 60 mph but i soon found out from my dad that people might get suspicious and the cops might get involved if i drived it on the road. So my idea was maybe to make a really fast bike!!Also I might go and look into chain drive bikes too if they are much more efficient.but i have also heard that chain drive bikes are complicated to setup and they are a hassle to length the chain is that so?? I think chain drive bikes might be nice because you have the advantage of toying with the speeds because you could experiment with different sizes of sprockets. Meanwhile I am enjoying building my first ever bike project!! which i have got much and most help and ideas from this website (Thanks!!) for the project I am using my purple 1994 specialized rockhopper which is a great bike to work with and I am hoping that my little 31cc ryobi engine and friction drive power will assist me well on my build.
(Im also keeping in mind that go karts also use chain drives)

I think i might go to chain drive and have some fun with that and i think ill get it in kit form so i dont have to hastle and improvise thing together which is whats is happening in my bike that I am doing right now. Ithink Ill order the kit off of zoombicycles.com they seem to have good prices and free shipping which is great. I think i shall go with the 80cc kit which says on the webpage that it can preform speeds of 38 to 50kmh which is pretty fast and another question is whats all these
shifter kits about? Zoombicycles Motor Bicycle Engine Kits FREE SHIPPING - 80cc Stealth Bicycle Engine Kit | 1-Bicycle Engine Kits | RuiHong Enterprises Ltd.
thats the kit i might buy from zoom bicycles in that link so check it out and tell me if its any good so yeh not 2 exspensive and it wont take me long to save up for it
(I think i might spray my bike black so it matches it all and my dads friend can airbrushes so maybe i cud get flames on the tank or somethin LOL that wud be pretty cool)

thanks all keep that in mind for my next build !!

My dad eased of the idea of not allowing me to make my gokart kart so any time soon
i will be making a go kart but i have the idea of maybe converting my friction ww engine bike to a
chain drive and see ho w that works