If the bottom of the bust is flat, you might be able to cast it as a one-piece mold. Just make a couple of relief cuts to the chin, etc., to allow demolding, unless the overhang forms air pockets, which then requires either a vent or a two-part mold. It might also be possible to cut it nearly in half, but not fully, allowing it to "hinge" open. That won't hold up for a long production run, but should be adequate for 10-20 casts; anything more and you might as well go two-piece. Smooth-on makes a clear mold compound, so you can sort-of see the piece inside where you're cutting.

If you're going with a two-piece mold, embedding it half-way in clay allows you to easily add "dimples" by pressing into the clay with the back of a pen or pencil, which will then mate with the other side and assist in aligning the halves.