Jon is the hardest part by far for bands to emulate because people know his voice better than the voices of the other instruments. However, that doesn't mean that you shouldn't get in somebody who at the very least does a half-decent job rather than merely looks like Jon if they put on a pair of sunglasses.

All of the tribute acts I mentioned above have good frontmen and Kadis, another tribute band I've seen videos of, also have a good lead vocalist.

Edit: Here's a couple of videos of Kadis

Prayer:

Blood on Blood: