Grandpa, before you lift a wrench or buy anything..........
Look up the laws of your state and how mopeds, and motorized bicycles are defined there.
This link shows each of the 50 states, so just find yours, click it and start reading.
Moped Laws by state
Now if you live deep in the rural areas or the have lots of land to ride on and want to let the
kids ride on your own property then that's different. But if you are where law enforcement
seems to have a quota to fill each day you'd better know what's up before you start.
Ohio, where I live, has a list of mopeds they approve. They are all factory built. No homebuilts.
You may be luckier in the state where you reside.
I've done loads of research where I live, and gone thru the Bueracracy of Ohio BMV and even
talked to my lawyer.
I bought the bike I've motorized in a yard sale for $5 bucks. After I restored it to what it is now
I've got about $50 in it and it's like new. But BMV doesn't like yard sale receipts. So I went across the river to a bicycle shop and got a blank sales receipt and filled it out to make it look like I
bought it used from them. That way it looks like it came from a business. This is the kind of
stuff you go thru. And like my Lawyer said, you could probably buy a used Honda 50 or some
thing for the same amount of money, get a title, license it and not have all the hastles.