but I have never heard of anyone simply replacing an inboard with a
normal engine such as from a
go-kart or Volkswagon

Why that complicated? I'd like to know why manufactures haven't
switched to installing B&S or Kohler lawn tractor engines in boats.
Just think when it blows up it could be replaced for $400-$600 instead
of the $6,000 that Yanmar wants. A 2cyl lawn tractor engine has as
much HP as a Yanmar. What's the problem?