Actually, there is an easier way, though. It's a glitch in the GTS. Offer Porygon in a trade in the GTS (for something like a Lv. 9 and under Dialga), and make sure it has the right item held for it to evolve. After that, immediately take it out of the GTS. If all goes right, it should evolve. That's how I got my Gengar.
But if you want we can still do it your way.