I cracked mine loose with a hand impact driver with a big phillips tip. Clutch heads on a striker are new to me, but bits are available as you see above. Give the impact driver a go if you have one, they will loosen it in small increments. I used them on boat motors all the time.
Here ya go.. Probably what Bobss396 used. https://www.amazon.com/ABN-Reversible-Manual-Impact-Screwdriver/dp/B07FDHGM6D Dave
That is it. I got mine at a cheap tool place 40 years ago, may have used it 10 times since, once was on my striker plates.
If you owned an old Japanese motorcycle with the soft Philips head screws this tool was a must have if you wanted to remove anything!
On outboard motors, same thing with all the corrosion from salt water. Looks like OEM builders never heard of never seize. The marine one had copper or something in it.
Do yourself a favor toss those screws in the junk and use these they hold better. https://www.ebay.com/itm/202751029148?fits=Make:Ford&hash=item2f34e72f9c:g:UykAAOSwqxddSaql