When I am comparing engines, I look at the manufacturers spec data for the various engine sizes and try to match these as closely as possible. There are many specs and not all specs and not all mfg's publish the same info, so I work with what I have and generally be conservative. I first try to match the horsepower ratings since this is an indication of how much power would be required to propel the plane. Torque and rpm come in next and then recommended prop sizes.