Allow myself to help.
Plagiarism is when a work is taken a sold without permission, whether edited or not. The limits are variable, as stated above, by the original artist's wishes.
However, even with permission, toribash rules state that one texture and recolours/variations of said texture are not allowed to be resold to others, the original must swap owner with or before edits. This was to try and increase the amount of creativity amongst the community and to stop some of the smaller scammers.
The idea is free to take, but remember its a concept. If you make a head too similar, they can say that the specifics of the head were mimicked and call a plagiarism claim - resulting in a ban.
It is safest to either attempt to purchase the texture you have your eye on, or make something of a similar genre that is irrelevant to the specifics of the head - like, the shapes, colours, concept, etc.
Anything more than that and I'd be nit-picking.