1st bobber build

Hey fellas. I just recently have fallen in love with the bobber, and I am very keen to start a build of my own, maybe a little Virago 250 or i bike that i have been looking at closely is the Suzi GS 550. Just wanna ask you guys what guidelines i need to follow to get it registered and what i need approved if I change. One of the things i will be changing is the Suspension on the back, from springs to solid struts. Thanks guys your advice is much appreciated