Scrub the rusted hole with a steel wire brush to remove any surface rust and corrosion, then spray some penetrating oil into the hole to help loosen up the rust.
Select the tap of the same size as the hole and seat the tap. Use the bolt that would be inserted into the hole to match up to a tap with the same diameter and number of threads. Since corrosion has made the hole unusable, start with a tap of the same size to see if you can restore the threads without increasing the size of the hole.
Place a drop or two of penetrating oil on the tip of the tap for lubrication and Insert the tap into the top of the rusted hole, making sure the tap is level in the hole.
Turn the tap a few turns, then back it out and inspect the tap to insure that it is level, and that it is cleaning the rust out of the threaded hole. Re-insert the tap and continue to clean out the thread. The tap will have a center groove to collect the rust and metal that it removes from the thread. Once you have cleaned the hole, remove the tap test to see if the cleaned threads will secure the bolt.