Notice how (in the pic below) that there is a stick retaining the rubber band on all four planes. I always install the ruber then finish the plane. And yes, when the rubber breaks it's time to build a new plane...
The question is are you building to fly?
If so the motor peg is not a peg as supplied in the kit. It is a piece of very light aluminum tube so you can hold the model in the winding stooge by placeing a piece of piano wire through the tube and into the hole in the stand. When winding you should always use a blast tube if putting in alot of turns. If the rubber breaks the blast tube prevents the exploding rubber from ripping the model apart. As to the replacement of a broken rubber..you either cut a small opening in the bottom of the model below the motor peg or you can make yourself a tool that is slotted in the end and push the new rubber towards the motor peg. I made mine out of small dia. plastic tubing, but you can also use 1/4 hard wood dowel.
