Forget easy out on b0lts that small or that rusted. If you can drill a hole for the easy out then you can drill it to clean up the threads.
First you need a good assortment of quality drill bits, a tap the size of the original and a hard center punch.
With a fine line marker mark an X on the broken bolt.
The center of the X is where you need to place the center punch and hit it with a hammer.
Drill a small hole straight through the bolt using the mark made by the punch to center the bit.
Once through you are almost home free. Keep drilling the next size up until you reach the outside diameter of the bolt without thread.
At this point you can run the original size tap through with some oil on it, if it gets real tight like its jammed don't force it just drill once size larger and try again.
The main thing to doing this easy job properly is center punching the bolt right in the middle and drill straight.
Keep oiling the drill bit as its working, spray oil Kroil, liquid wrench works okay.
Work slowly and carefully with precision, this is not hack work. When done properly you will just leave the original threads intact.
The first few will be a PITA but gets easier with practice, i do this many times a month.
I just did 6 exhaust pipe flange to manifold studs on an angle with an angle drill and did not use an oversize stud.
WEAR SAFETY GLASSES!