RE: 120 decreasing rpm
Not really, mine looked like new when I discarded it, they wear very evenly and smoothly, I really doubted if I needed to change it, but based on the behavior I did and fixed,sorry I can not have better news on that. I'll get a picture of the discarded piston tomorrow, I still have it.
The reason why I changed the piston is because there was a new version of that piston which produced more power, but try changing the ring at least, and make sure the piston grove is very clean.
Regards