This is the correct part, steel with rubber bonded to it. Since your motor is non-stock I am unsure how tight it may seal. The original fits like a boot surrounding the motor. I am also unaware of a reproduction for this, post in the Swap Meet someone may have a spare.
https://www.flickr.com/photos/advance-design_parts_co/8129342505/in/album-72157631868452754/
https://www.flickr.com/photos/advance-design_parts_co/8129343557/in/album-72157631868452754/