If the cup is aluminum, it's unlikely to have damaged the steel steer tube threads. This is a Good Thing, as it's generally easier and less expensive to replace the headset than to replace the fork. There are plenty of threaded headsets still available, e.g. the Tange "Levin," Velo-Orange, etc. Tange even makes a threaded headset with roller bearings in the lower cup for better handling of the thrust loads headsets must take in use. Keep the new headset properly adjusted and the threads will stay intact.

The binding the OP describes is indicative of the cups not being perfectly parallel, either because they are not fully seated or because the head tube needs milling and facing. If you have a cup press, you can try re-seating the cups. If that doesn't help, it's time to take it to the LBS for milling and facing.