You can add a gasoline motor to a bicycle, but you can also retrofit an electric motor. It's easy to google either option. An electric retrofit may even escape notice, as the motor is in the wheel's hub and the battery is on a bracket behind the seat, and obviously they are quiet.
But as someone said earlier, you aren't likely to go into town with an electric retrofit, because you may end up peddling your way back. OTOH, people are used to seeing and giving way to bicycles so maybe the road rage incidents wouldn't be an issue, versus a slow scooter.