You did verify that it was the headset, right? Suspend the front of the bike off of the ground. It helps to have a repair stand, but you might be able to improvise something if you don't have one.

Take the front wheel off. Grab the *end* of one of the forks where the front axle goes. Feel for play. Rotate the forks and repeat the test as you rotate. If you're not feeling any play, then it might actually be the front wheel hub that's loose.