Consider that until about 2003, you could go down to your Chevy dealer and buy and engine with some parts capatibility with an engine first produced in 1955- the Chevrolet small block V-8. That is a 48 year production run. Now consider that from 1918 to 1942 is only 24 years.
What about those flat 4 and flat 6 aluminum light aircraft engines? Lycoming built the first first O-145 in 1938 and something basically very similar is still sold new.
Much easier to use an engine with the right basics and improve it than to start with a clean sheet- that big PT boat engine apparently got the basics right.