you can also explore using friction-drive parts as an easier route than mixing and matching wheels & sprockets & whatever. you can find ready-made clutches & rollers that'll bolt right on to these engines.
and just so you know:
nice looking scooter would
this be a good engine?
that 47cc engine comes with a bowl carburetor and requires a gas-tank that's higher than the engine for gravity-feed. you can buy diaphragm conversion kits, but they will double/triple your final cost.
the other style, 22cc-49cc, uses a diaphragm carby, and you can place the tank anywhere. you can also get versions with an attached tank.
for reference only, not a recommendation, just a good look at your options:
http://www.partsforscooters.com/All-Parts/22cc-49cc-2-Strok-Engines
PSA: it's my small-engine opinion that what you're looking to do can be accomplished with as little as 22cc, especially if you have an eye open for legal & social tolerance. be mindful that gas motorized bicycles have become illegal in WA because of abuse, you wouldn't wanna let that happen to scooters too.