The plastic pull-off is not a replacment for the metal one. The arm is too short and not enough travel. It can be made to work, but will require bending of some parts and comprimise in choke function.
To remove the metal one, carefully spread the arms and clamp the bracket in a soft jawed vise. With a small hammer and punch drive the old pull-off out of the bracket. Install the new one, and gently close the arm just enough to retain it with a pair of pliers.....Cliff