Take a gander at this.

[Jim Brierley]

GPierce has it right, the engine is installed "backwards" so the prop is driven off the back of the crank, as in the Model A car. I think more A engines were used, because of costs, back in the day. A friend of mine had a 'T' powered Piet, the mag was driven by a bicycle chain, directly off the crank. Mags are designed with internal reduction gears, for increased RPM for their generator.