If Revell still has the Monogram B 52 in production, you can contact their customer service and request a replacement part.
Check this Revell web page for instructions on how to request a replacement part:
http://www.revell.com/support/index.html
Revell has helped me out on more than one occasion when I needed a part, even though the loss or damage was due to my error.
They also ship the part to you at no charge.
Of course, I am presuming you would welcome a kit part and are not looking specifically for a vacu-form replacement