Law / What car modifications can be performed on a car registered in Poland? [6]

Hello,

Since lot of cars get modified here in Poland, I'd like to know if the car I purchase is legal or no.
In details:
1. Can the engine be swapped without limitations?
2. If no, can the engine be swapped for another one from the same car model but with different size/capacity/power? For example, the engine from a mercedes w203 c230 into a mercedes w203 c200

3. Can any other mechanical part be swapped? Like take a transmission or a differential from a thir party car parts builder company and put it into the car.

4. If no, can it be done using the original car manufacturer's parts?
5. Can the front/rear bumpers and sideskirts be swapped using non original parts?

I'm a little concerned because there are lot and lot of old cars that may have been modified with stuffs that in my country of origin are considered illegal, so I don't really want to have trouble with insurance or yearly car inspection.

I know these are not easy questions to answer but my limited polish language skills don't allow me to get all the informations I'd like on polish law.

So thanks in advance for any answer.