I'm not exactly sure how your setup perhaps a quick picture will help but this one should get you extremely close the critical things are the fitting sizes....
Is your sump basically a plastic box with bilge pump mounted inside? You may be able to get the model number of the pump off the old one if that is the case, not really sure how yours is set up though??
Thanks guys. I found a new attwood that fits. It's the standard box, which was more difficult to find than the "larger box" that didn't fit my opening.
Safe boating!