Rather than one of the brothers, I'd suspect that their unsung partner and chief mechanical designer, Charles Taylor was responsible. He built the engine in six weeks in the brother's absence. https://en.wikipedia.org/wiki/Charlie_Taylor_%28me chanic%29
Smithsonian's Paul Garber always spoke highly of Taylor, and interviewed him extensively. Taylor hand hacksawed the crankshaft out of a plate of steel, and machined the journals. The whole engine is an exercise of pragmatism, with no time spent on frills or polishing. Lots of raw material surfaces and saw and file marks still evident.
The fuel system is another example of simplicity and ingenuity.
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