Gears are what makes the difference. If it has a manual transmission its a motorcycle in just about every state. If you plan on driving it like a motorcycle, then it should be registered as one. One day you might get a ticket for going really fast and having the motor not match the vin...they might catch you if you're in a state where everything has to be registered. Cops dont care about the laws, they write tickets and let you work the rest out, expect tickets if your bike is turned into a motorcycle.
As far as advice, dont do it...you're going to ruin that bike and waste lots of time and money doing it.