OEM turbo engines have lower compression pistons so that there isn't too much pressure in the cylinders under boost. A stock NA engine has higher compression pistons so if a turbo is added, one would have to use very little boost or the engine will soon be damaged.
The OEM turbo cars also have "beefier" trannies so a modified engines trans might not hold up either.

It's far better, easier, cheaper to sell your NA 850 and just buy a 850R instead. On the used car market today, the cars are probably similar in price so it shouldn't cost much to switch cars. The OEM turbo car is also modifiable so one can make it faster, safely (no serious damage). The aftermarket turbo will be expensive to do it right, probably will have limited enhancement due to needing very low boost and is likely to be very problematic in comparison.