From the pictures it looks like the cracks may only be in the stucco, and the underlying structural wall may be unaffected. It's hard to tell without actually inspecting the wall, but my guess is that these cracks are only skin deep. However, you will want to treat them to prevent them from spreading.
Here are some excerpts from the article.
All buildings are subjected to a
variety of stresses in various forms
and this stress is often transferred
to the plaster stucco membrane.
Stucco
is not and never was intended to be a
structural material.
Cement stucco is
a relatively thin, brittle shell over
a framed structure and can crack if
the stresses are transferred and
overcome the strength of the plaster
membrane.
SHRINKAGE STRESS
All Portland cement
shrinks as it cures. Controlling and
minimizing that shrinkage is critical
to minimizing cracks
Shrinkage cracking
tends to have a map-like cracking
pattern.
OUTSIDE SOURCES
Structural loads,
lateral force racking, thermal
changes, wind, lumber shrinkage and
seismic events are all examples of
outside sources of stress.
HOW HARMFUL IS A HAIRLINE CRACK?
a hairline
crack does not automatically mean the
stucco is faulty, failing or will not
function for the service life of the
building.
MAKING REPAIRS
Once the decision is made to make
repairs, there are several options.
Option 1: Treat the joint with the
same finish material.
Option 2: Paint the wall or ceiling
after treating the crack with an
acrylic paint or paint designed for
stucco.
Option 3: An elastomeric paint
coating.
Option 4: Recoat the wall from
architectural break to architectural
break with the same finish material.
Option 5: A new basecoat and mesh over
the wall area.
Best Answer
You should be able to get a replacement shingle at a lumber yard or home center. They may sell them solo or you may have to buy a bundle. Looking at the photo it is hard to tell the style but in my locale they come split or sawn. They are installed either sanded or rough side out. Pick a shingle with the best appearance match and wider than needed. Trim it to fit with a utility knife including the angle cut at the top. Ideally you want the top of shingle to be under the trim. If the old shingle fell off rather than broke slide it under the trim. Stain both sides and the edges of the replacement shingle. To attach the shingle use exterior adhesive or drill a couple of small pilot holes in the face of the shingle. Use 4d stainless steel nails to attach it to the sheathing. If you were unable to slide the shingle under the trim put a bead of chaulking where the shingle meets the trim.