Catalina Direct has a Bomar hatch that sounds like it may be what you want, I'd call to make sure, seems like there might have a couple different sizes used at that time. I've attached link for the CD page here:
http://www.catalinadirect.com/index....Product_ID=730
And the link to Bomar's (Pompanette) page of trapezoidal hatches, I'd call them too to see which one they think is a direct replacement, and shop it around for your best price.
http://www.pompanette.com/pompweb.ns...a?OpenDocument