Odd You Should Mention This ....................
because I am presently looking at installing a blower
into my engine compartment.
Perhaps I should mention that my engine is stored in a Nike shoe box under the companionway stairs. It's nice, neat, compact, and pretty but also causes a few problems ............ the biggest being the 145 deg. F air the poor old Yanmar has to gulp down. The bottom of the shoe box is wide open to the cold bilge for air. but the engine only requires 30.3 CFM when running at 2600 RPM.
When I jury rigged in the temporary Shurflo Yellow Tail exhaust fan, the interior temperature dropped to 80 deg. F. The fan removes about 100 CFM + the engine intake = 130 CFM.
This may have been the reason for the original installation of your fan.
My only problem now is to figure where

I can discharge the fan without having to put up with the noise, hot air, possible ingress of sea water, hot engine smell, occasional flying engine parts, etc, etc, etc.