I don't even see what the point of putting a gas engine on a bicycle would be. It's the worst of both worlds. You're still a noisy, polluting stinkbox, you don't get any exercise, you can't listen to either natural sounds or your own music/audio, and you can't go fast. You might as well drive.

Ebikes at least make SOME sense, though I wouldn't own one since I'm in it largely for the exercise.