Sorry if this doesn't help at all. ; Just figured I'd get it out there.
Bronzong @ Leftovers
EVs: Def 255 / Sp. Def 255
While Bronzong does sound like a good replacement for Porygon-Z, the set and its EVs are out of place. First, you'll have to max out its HP followed by the defenses, and any offensive stats, if possible. It also has to have a Relaxed nature to make Gyro Ball hit harder. What I mean is seen here:
-Hidden Power (Ice)/Gyro Ball
EVs: 252 HP/84 Atk/80 Def/92 SDef