Since you're new to this, you may want to first become familiar with your State's laws.(if you haven't
already) That way whatever you buy and build will be something the State will have no issues with.
You'll be able to get the sticker ( good for three years) which denotes your bike is a "moped" and there by exempt of motorvehicle regulations. (and having to carry insurance)
Here in Ohio they want proof that your bicycle was purchased "ligitimately" and wasn't stolen. (in my area they want a receipt from a WalMart or bicycle shop etc, where as flea markets and yard sale poof of purchase aren't taken very seriously. You wll need to have a receipt for your motor as well)
Since we build our bikes from parts we don't have a "Manufacturers Certificate of Origin" like a new car or motorcycle would have. It's a headache for the bureaucrats at the Bureau of Motor Vehicles
and they will likely want to contact the Capital Building HQ as to what to do with your motorbicycle.
I told my local BMV that I only took on the project as I could get 160 mpg with it. (that impressed them last summer when gas was $4.05 a gallon and was much of the reason they took me serious).
But once you know the legal parameters you can build in, then you can go to to sellers web sites
with more certainty and confidence that what you buy and build will qualify for a license so you can
ride on the road without each and every law enforcement officer pulling you over everytime you're