The turbo engine has different pistons, sodium filled valves,
oil cooled crank, lots of check valves, smaller displacement
engine (for thicker cylinder walls to handle the heat), lower
compression, different ECU, different exhaust, coolant routing
to get coolant to the turbo and back to the engine, ditto for oil,
and many other mods. The premium they charge for the turbo when new
is really a good deal -- it is well engineered and much more then
a bolt-on to the normally aspirated engine.

When this question comes up (every few weeks) the general reply is
that it is cheaper to just find a turbo. Or, for those that own
the car already, it is cheaper to sell it and but a turbo model.