How do i change a rad on a gravity fed system?

i have a gravity fed system and need to change a rad i have changed rads before filling the system after by the filling loop but i cant find one on this system,and putting the presssure up to 1.5 so does someone know how the gravity fed one works? also i need to change the loo but the waste pipe coming in is an old cast iron one and is massive, how do i cut it down and fit a flexible pan connector