You need a piston kit, and a top-end gasket kit, cost of around $100 for a 125. The piston kit usually includes the piston,ring and wrist pin, the gasket kit includes the cylinder base and head gaskets, new pipe o-rings and anyting else particular to your model and year.
Only if it is way out of spec or extremely damaged. You should take some measurements before you put it back together to determine what you need to do. Does it have a sleeve or is it plated?
You are riding an old bike. You would be miles ahead to purchase a book like Eric Gorr's, Motocross & Off-Road Performance Handbook.
He covers just about all the issues concerning top-end rebuilds with narratives and pics.
The book also covers many other area's off bike maintenance and is, IMO, invaluable to the novice wrencher.
A very good investment.