Kindly allow me to offer my two cents on the subject. How much it will cost depends upon how authentic you want your bike to look.
For budget minded approach you can get clipon handlebars, Ravi piaggio's seat, fuel tank and side covers. With all that you should also consider a sports type headlight (hint: unique UD100).
Install some fat tyres and alloy rims and bike should look something like a sports bike.