AK Bolt/Piston Compatibility/Upgrade Options

#1
I was visiting my father last week and he took me out to shoot his Romanian wasr-2 that he's had since 2007. He mainly wanted me to help teach him some fundamentals of zeroing and handling of his rifle, since the last time I had been shooting with him was before my own training in the military and also with the AK platform. He has never been to any sort of firearm training other than basic firearm safety way back in the early 70s. While observing my dad during tactical reloads, I noticed he was far more fond of reaching under to manipulate the charging handle than reaching over. In spite of his charging handle being swept up, similar to the aims-74 style charging handle, which is obviously meant for reaching over the top. At the end of the session I also taught him how to fully field strip and clean his rifle properly. That's when I learned that the piston had been neglected and was severely pitted and covered in very thick carbon build up from over the years. Thus leading to my main question here...

TL;DR

Would I/he have any issues replacing the entire bolt carrier/piston in his wasr-2 with a new one say a Bulgarian 5.45 bolt/piston with a straight charging handle?

The main concern is the possibility of any malfunctions or hang up's. I understand fully that carbon build up is actually not really an issue with an ak. It's just more of an incentive to get a more ergonomic option that both he and I agreed on would benefit his tactical reloads. I also have heard and read from several sources that most ak parts are very interchangeable from various national manufacturers. I just wanted to hear if there were any other solid opinions that would lean towards one bolt/piston vs another.