Nathan, welcome to the forum. Commendable that you want your grandson to experience the fulfillment of building and enjoying what he has done. He'll always remember it. As for you...lol, well sir here's a warning. Motorized bicycles are addictive and if you build one, you'll want another, and then another and so on until you run out of garage space.
Have fun and ride safe. We're always here with answers if you run into problems.
Tom (another old drag racer, B/SR A/FD)