When I bought my previous evo back in 2000, I found that one of the front cylinder exhaust flange studs was busted off the same as you describe. I tried the ezi-out trick with the same results as you had!! Fuck it!!!!

Ended up just using the one stud, (with a spacer) to secure the header. In much the same way as the shovel heads did.....Lasted fine for about 8 years till I pulled the heads for a top end re build. Took the head to a shop to have the stud/ezi-out removed. Had to be done with a dremel tool and took a couple of hours (apparently!) and I was charged accordingly!

End result was it was fixed with a new stud inserted. 

Rebuilt bike after tossing up whether to replace existing studs and deciding against it due to not wanting to break another.

You guessed it!!! Not long after.... found one of the rear studs sheared off in the same manner!  FFUUCCKK!!

Needless to say......I didn't go near an ezi-out this time.........just secured header with one stud as before........as far as I know it is still holding in there!.........sold the bike a while back.

(Sorry new owner!..if your reading this!)

You could also try welding a steel washer with the same inside diameter or slightly bigger than the broken stud with a mig and then welding the tallest and biggest nut that fits onto  the washer 

then using just your ratchet and a socket to suit the nut the bigger the better

soak with penatrene overnight

ps a mig in my opinion will give you far better penetration and no slag......