One source I've read mentioned that B&S vehicle engines have a spring
for the throttle so that when the throttle lever or peddle is let go,
the engine returns to idle, ala car gas pedal. Most are intended for
applications like lawnmowers where when you set the throttle, it stays
set. One parts list had the callouts for what was needed to convert one
particular engine to a kart engine. It might even have been on the
Briggs site, I was looking for governor linkage diagrams at the time.
There's a lot of differences in the different series of engines, some
minor, some major, like cylinder material. I've got a book that
details some, but not all, the differences. Some models have just
sleeve bearings, some have partial ball bearings, some have full ball
bearings. Some are splash oiled, others have partial or full pressure
oiling. Without a catalog or some other factory-supplied material, I
don't think it's really possible to just look at an engine and tell
what's inside.

Stan