If I understand your point, that is because sometimes the included components for an engines published weight are not listed. A published "empty weight" for an engine is not meaningful unless you know if that includes exhaust, radiator, alternator etc. Then comes fluids for the installed weight.What no one seems to publish is the plane weight with a Rotax compared to Aeromomentum or Viking.
Makes comparison for a prospective buyer more challenging.
If you know the fully ready to install and run engine weight, in that case where all the components are the same, you don't need to weigh the aircraft to make a proper comparison.
That is why I looked for complete installed weights with all fluids and all accessories excluding only the propeller for comparing the Rotax 912 ULS with the 90HP 3cyl Viking conversion.
Rotax 912 ULS Installed Weight (195#)
Viking 90HP unbolted and suspended just after a flight with only propeller removed. (193#)