this is a hex head sheet metal screw
Is this the type of screw you are reffering to?
The next question is are they accessible? OJ I see that they are the blade screws. Here's another trick. Find the next size smaller hex drive screw, either Metric or SAE and hammer it on if necessary to get a good bite.
Take a Dremel and make a slot in the head similar to the one I posted.
Take a small chisel and drive it at an angle counter clockwise to loosen the screw.
Grind the head off the screw and center punch the remaining to drill out the existing screw.
If all else fails take it to your friendly machine shop who will know exactly what to do.
Replace the screws with Allen head machine screws.