JStricker wrote:
Long stroke, slow turning engines ARE still used and designed all the time
in agricultural, industrial, and marine applications. It all depends on
where you want your peak torque at. In an aircraft engine, the best prop
efficiency trade-off is in the 2200 - 2500 rpm range and the most efficient
way to transmit power is via direct drive, hence engines are designed for
the applications in which they are used.

Where are the long stroke, slow turning gas small boat engines?

In museums. Inboards ALL use autoderivative engines and ahve for
almost 50 years