That isn't a flywheel. It a ring gear mount for the starter
that also drives belt for the alternator (if it's a Lyc). It's aluminum
and weighs about four pounds. An engine such as the O-320, with its
four huge cylinders (compared to a Chev 350's eight much smaller ones,
for instance) would need a wheel of about 50 pounds in that small
diameter.
In all direct-drive aircraft engines the prop is the
flywheel. In most geared certified engines, the prop is the flywheel.
Geared ultralight engines like the Rotax have a separate flywheel, as
do many of the converted and geared or belted auto engines. They have
other problems with resonance between the prop and flywheel that can
lead to belt or gear failures if not properly designed and operated.

Dan