I think you would be better off with a gze. The 20 valve is not mutch of an upgrade from the 16v for the money you would have to pay to get it.
You could get the gze for roughly the same price as a 20 valve and have way more power potential. In saying that I havent seen a gze motor for sale for quite some time.
You may want to look at getting a half cut or an engine package from places like adelaide jap etc etc.