RE: Smallest gasoline engine?
Before Ray Arden invented the glo plug all model engines were gas, I still have two engines manufatured by Arden that are .09 and .19 cubic inches. The Ardens are the smallest I ever owned but also had an Olson .23 several McCoys, a Brown Jr. and others.
What the smallest manufactured now is I haven't a clue but know that weed wackers have been converted to model use and some of them are small.