Again, weld a nut to the sheared off easy-out and remove it. I cannot see which type you are using, I do not like the left hand thread tapered type, as you screw them in they squeeze the nut/bolt out making it even tighter in the casting. Go too far and you will crack the casting. There are parallel easy outs that work well with the correct drill but once they slip you have to go up a size.
I feel attempting to drill, grind and then picking out the remaining thread should be you last idea as you will find it very difficult to drill central and square. You could well end up using a thread insert but that opens up difficulties with making it oil tight as you may encroach on the o'ring.
Of course removing the head allows you to get to a machine shop for a professional repair, at a cost.
Please try the welding a steel nut on first, it really does work and is cheap.