Inza Nelson studied the mystic arts with Kent, so even before she donned the Helm, she also had a small amount of mystic might. And both she and Kent studied the fighting arts. Furthermore, people forget that the Amulet of Anubis and Cloak of Fate also contain mystical powers. So although the Helm is the main artifact of Fate that provides his powers, it is not the sole source powers.
Hector Hall, being reincarnated several times, and being a crime fighter in former lives as well knows how to handle himself in a fist fight. Even Khalid, the newest Dr. Fate, appears to have a connection with Nabu even when he does not have the Helmet on. How deep those powers are or will be without the Helm still remains to be seen.
So to those who say that Dr. Fate ain't much without his helmet I say SUCK IT. You can pretty much say the same thing about Green Lantern, Hourman, Iron Man, etc... But that is what draws me to these type of characters, anyone can be Dr. Fate or Green Lantern. That is a very cool concept.