Change only one piston if others are good.
But usually opening the engine costs.
When its open, you change gaskets, bearings and stuff...
Stock pistons are aluminum and are good if you dont rev too high.
Yes, you should change the cam and lifters. Problem is low comp ratio.
Hotter cam needs higher comp ratio and stock comp ratio is low for everything.
Clean engine(inside) dont need oiling mods.

Hyvinkää, FI