Some time ago I wanted to include in my late Roman army a unit of ballistarii, or crossbows... As far as I know, there are not specific miniatures for the "rare" late Roman crossbows, so I needed some conversion work... How? well, I took 4 Psiloi of "Crusader Miniatures" Byzantine range, and substituted their heads them with Roman/Arthurian heads from "West Wind"... and voilà, sort of a Roman ballistarii.
I used as reference some Late Roman illustrations from several books (Osprey, Concord, etc...)
And now, just a test model for another simple conversion that I will cover in another post: Late Roman "Funditores" or slingers. I use Gripping Beast plastics to convert these, very simple, and painted in a "fast" way.