i think I know the style of impact screwdriver you are talking about LG, I have 2 of them and they work well. i couldn't get the link to work. I think the screws are too close to the crank gear to fit the socket that holds the srew driver bit, unless the crank is out I suppose. thats why I mentioned to use a screwdriver and a vice grip, same theory. I have had good luck with the method I mentioned, even on really big bolts found in the heavy duty equipment world and especially on bolts that were loctited in place. of course if there is room to heat the block in the area then that works well too but I hate to use spot heat on cast iron because sometimes the iron can crack around the heated area due to uneven heating.
one thing I wonder about is whether the crank can be removed with the plate still on. if so, check and see if there are dowels holding the plate as well, for lining the plate up in a certain spot. if not, and there is only a single screw holding it, possibly the plate could be rotated slightly and that may also start to turn the screw head. not sure whats on the backside of the plate so dunno if that would work.
I have found that heating the head of the screw/bolt and allowing to cool seems to work most of the time. I think the theory is that the screw expands fastee than the block and then shrinks a little smallee than it was before. like welding sheet metal and seeing how it shrinks when it cools. that seems to break loose any rust or loctite. then striking it several times helps even more and using torque while striking will usually do the job. worth a try if you don't have an impact screwdriver
try this link for an impact screwdriver
https://www.amazon.com/TEKTON-2905-8.../dp/B000NPPATS