I believe you can find bolt on kits to add gas or electric motors to a bike. Many of them work by using a friction wheel on your rear tire, so expect some additional wear on the tire.
Before you spend too much money on this, consider the local laws in your area, and if it's technically illegal to operate the bike on public streets once its modified.
You can buy a used 49cc scooter for around $600 that will run so long on a tank of gas you'll forget where the tank is.