This question comes up here on a regular basis. The general
concensus is it is cheaper to sell your car and buy a turbo.
Will also be much more reliable (and resale value on a modified
car would be very low).

As far as parts, changes to the turbo include: Forged pistons
giving a lower compression ratio, sodium filled valves to handle
the heat, oil cooling for piston bottoms (oil is sprayed on the
bottom of the pistons down in the engine), ECU, a whole bunch
of check valves, exhaust manifold, exhaust plumbing behind the
turbo, an intercooler, and air intake plumbing is different. The
transmission might be beefed up a bit (manuals are, not sure
about automatics). Different spark plugs. Fuel injectors might
have a higher flow rate (not sure about that).

In all, quite a few changes. I have a T5 turbo and a normally
aspirated model (GLT) and opening the hoods you can see there are
many small differences as well as large ones.

There is a company that makes a kit for the 850 normally
aspirated models. They advertise in European Car magazine
and are based in Germany (as I recall). Sorry, I forget their
name.