If the abutment / crown are well fitting and immobile, treat the restoration like a natural tooth: Attempt to prepare it for a crown and impression it like regular C&B case. Half way through, the existing crown can be delaminated from the underlying abutment , reducing the need to fully prepping the metal. By then however, you may find that the vibrations and “heat” of the preparation have made it easy to turn the screw with lower pressure!
If you must remove and replace the abutment: While the screw head is not stripped : You should be able to use whatever torque wrench you are using to put as much pressure as your hardware can take. Of course you are keeping a good apical pressure while forcing it anti-clockwise to avoid stripping the head.
Most likely you will be able to unscrew around 40-60 NCm.(varies greatly depending on specific screw properties) Beyond that you are risking separating the head of the screw or stripping it depending on various mechanical factors.
If the head is snapped: Your abutment will come out easily and you should very very carefully, with good magnification and illumination, using an explorer, turn the body of the screw, which should be loose and kind of wobbly at this time, anti-clockwise to remove it. If you strip the head: Use a number 1 round FG long shank bur in high speed with good irrigation to grind off the head of the screw until abutment is released. You will most likely need a new abutment after this procedure, but your implant is 100% protected and unaffected by your actions. I just posted this on another thread.
When you have a broken screw, typically, if the screw is correctly engineered and matched with the implant, when over stressed, head is separated before the threads start with some of the body (as little as .2mm )sticking out. Again if properly matched, no matter how tight it was torqued, the screw body would be loose in the implant after separation..
You should be able to turn the loose screw body by tip of an explorer until the body is out. Use illumination and magnification for this task. If the screw is not a perfect match, and it has bottomed out , or it is not loose when the head is removed, then, you are screwed. Not easy to remove the bottomed out / tight screw without substantial damage to implant.
In this case, you may want to consider placing a stronger implant and use original parts for restorations.