If this is String trimmer/Brushcutter setup it is a T20 but most of the T20 drivers I able to get will not fit through the neck of the clutch drum and would need modifying to do so which have done as I do use it a lot. Now at one time they did put red Loctite on the threads and if so it will be a pain to get loose. Sometimes you can shock it loose by the bit in place smack it with a hammer. But before you do this get a light to shine down shaft and what damage you have done to the screw head. At this you might fine that there is slot in the head too; sometimes there is one, other times there is not. If so then a hollow ground flat tip will work at times. Even this type driver you will need to grind the shaft above the tip to get it in place. as you will want to have as much of tip engaging the screw as you can. You want the flat tip just narrow enough to pass diagonally thru square neck but shaft will need to even narrower to pass the square shape.
Sorry I can't give the exact of the flat tip tool as I lost mine in the 2014 move but at least I remember how I made it if I ever need it again.
