How do I revert the city portal back to its original state after it was changed to fire magic?

You're an interesting person; you installed a DIFFERENT VERSION of the game and want it to be "like it was before." But it won't be "like it was before." There's not just a portal in the fire; there's also a dimensional door in the water, a slowdown on the second level, an increase in Phoenixes to 3, and many, many other things – it's impossible to list them all.