This prop is driven by hardened gears cut by Les and the engine is cooled by a water scoop, piping being carried inside the lower casing. A brass water jacket was shrunk on the cylinder after the air-cooling fins had been turned off.
Water is ejected at top rear of the lower unit, just as in the full-sized outboard engines.
Finished engine is just about twice the size of commercial .049 units, as illustrated by Allyn Sea Fury outboard in upper shot. It is from a July 1954 Popular Mechanics Science magazine. Here are a few articles from Model Airplane News and Air Trails. Here is a parts list for the Atwood outboard. This is a big file, saved as a Tiff, but it has exceptional clarity if you print it out.