Why bother sinking money into rebuilding a NPI? If it were my money, I'd pick up an engine from a 3rd Gen Ford Exploer (2002-2005). Same PI engine engine found in every other Ford after 2001, with the benefit using an aluminum block. That's a 70lb weight drop in the front for no added effort. The Explorers are so overlooked, you'd probably get it for a song.
Not only will it make more power over your current NPI engine, but it'll improve your MN12's handling as well.