You can try using penetrating oil on the screw. Make sure you don't get any inside the cylinder, or at least don't turn the crank if oil does get in. You may also find using the "anti-freeze in a crock pot" method to clean the engine will also loosen the screws, as a combination of the cleaning/heating action.

As a last resort, you can drill the head off the screw. Be real careful not to drill too deep and damage the head.