Would you prefer to save the old head if possible?
If not, then as mentioned a sawzall or even a jigsaw is the way to go. However, if you'd like to save it I'd second Mike Reeds suggestion for cutting an access hole which looks like it may be needed anyway when you have to deal with the plumbing
after you remove the bowl.
At the same time, also as mike says if you lift
that floor grate out, can you reach the bolts that way?