David Hill wrote ...
I'm working on a replica of a 1924 airplane originally powered by an
Indian Chief motorcycle engine. I've spent a year or two trying to find
a modern engine that somewhat resembles the Indian, and the best I have
come up with is a Harley-Davidson.

Why not a current production Indian? Can't say that I know anything
about motorsickles, but I noticed one of the new ones a couple weeks
ago & wondered.

... I can get ~40 hp at ~3400 rpm
(direct drive) or ~60 hp at ~4500 rpm (PSRU setup).

Better check your math (PSRU = more torque @ lower rpm, but the HP
stays the same).

Daniel