Muffler Swap

[Rookie Rider]

Hey guys, i know this is an old thread but im gonna try to ask anyway. lol, i dont have a micrometer or caliper so does anyone know the exact size of the outside diameter of the pipes that come out of the motor on the gz? And can i just put or slide a new muffler on to the original pipe after cutting off the ugly original muffler? And whats a reducer and what does it do ?
sorry, i dont know much about cars or motorcycles. Please help. DAVE.