Drill it and easy out. DO NOT BREAK THE EASY OUT. You will be screwed then. Only way to get a broken easy out "out" is to torch it out. Left hand drill bits work sometimes. I have welded a nut on a broken bolt and then turned it out. The heat sometimes helps alot with a stuck bolt. But this method might not work on this depending how much bolt is sticking out of the crank boss. You are most likely screwed if the easy out doesn't work.
Merry Xmas and good luck!
PS you could always leave it that way and trade it off. You would be suprized how many screw-ups this like this I have found on used cars.
leave it and trade it off? so it's someone elses problem? no that's not the right thing to do. get it out of there. might have to take it to a machine shop. either way, sounds like a problem i'm glad i don't have.
if you have access to a welder that will be the easiest way to get out, even if it's stuck inside the crank, just throw some weld on it and either build it up till it's sticking out and grab it with vice grips or weld a nut on it, that's how i get broken exhaust studs out of 5.4's.
if you cant weld it, try a left handed drill bit first as it's the less intensive route, and flywheel bolts aren't usually hard to turn when just threading in, so this might work. then i'd do the easy out but dont use harbor freight easy outs, craftsman at the minimum.
Myself or anyone else I know has never had success with an easy out, I wouldnt even bother. On thing that might work is a left handed thread. Drill and tap with a left thread and use a left handed bolt on it, all found at McMaster-Carr
As you turn to the left it will be tightening down on the bolt and should loosen and remove the remaining threaded portion.
Or if you have access to a welder weld a nut ontop
Also if you remove the flywheel and you have a decent amount of the stud exposed you could give it a go with some vice grips.
Make sure to PUNCH the center of the bolt and make sure it is DEAD center before you begin to drill. Start small, use cutting oil, and go nice and easy. Slowly increase sizes. Breaking a bit is just about as bad as breaking an easy out so be very careful