The standard generally is whether the trademark would cause confusion to or deceive consumers. Naming a company after an existing brand which is an invented word (being a corruption of googol) would fall under the latter, and intent matters too.
As to the former, trademark classes are one of the ways in which confusion is determined, but it’s not the only way for sure.
That said, if it’s a word borrowed from another language and there’s no intent to deceive, I imagine courts would allow it - see also Apple/Apple Corps, and the many companies called “Delta” not in the airline business.
The Vespa is an iconic and well known brand in Italy and worldwide. If Piaggio isn't litigating it, I wonder what their lawyers are even doing.