Can I replace the HDX screen with an HD Screen?

As between the Kindle HD 7" and the HDX 7, the screens are completely different.

You should keep looking; HDX screens are readily available on sites such as eBay and AliExpress. However, be forewarned: (1) in most cases, screen replacement is very challenging. You must be technically inclined and have the proper tools! (2) there are several models of Kindle Fire and they each feature TWO screens: a digitizer and an LCD -- be sure to get the correct screen, both as to function and model.