I'll expand a little on what I just posted; and more!
A 75mm enlarger lens on tubes or thin bellows will the the cheapest and sharpest headshot portrait lens you can imagine. Macro lenses in the 90-105mm range are exceedingly sharp too, but rather more costly. A decent 1.4x TC on your FA50/1.4 will become a 70/2 that should do great for slightly softer portraits. Hmmm, and my two-buck Tomioka 55/1.4 would become a 77/2!
And adding a thin (10mm) macro-tube ring to a 100 or 135 brings the far-focus WAY in. I just tried that with an M42 135; infinity is now a little over 2m, for good sharp headshots. (With that ring, focus range on my ST135/2.5 goes from 1.5m-->infinity to 1-->2m.) Similarly, for softer shoots, a +1 diopter closeup adapter on any lens brings the focus range down to 0.5-->1m, for VERY THIN DOF even on slow lenses like a 50/4.
There are many many ways to use lenses for portraiture. I like to think cheap.
"There's more than one way to skin a cat," she mused,
as she pinned its little feet to the dissecting board.