I've never heard of kroil. I might have to give it a try. What I've been using is liquid wrench. It does a fairly good job but it has some strong fumes.
The screws that gave me the most grief were the 2 screws on the front cover and the two screws on the chromed part behind the bobbin winder. Those 4 screws were rusted in the threads and rusted to the covers so I couldn't turn them out. I thought of a genius idea that worked very well to get those parts off. Both the front cover and the side piece need to be replaced so on the front cover I cut out a square around the bottom screw with a die grinder. Then i bent the cover enough to clear the bottom square piece and the leverage was enough to turn the screw and then free the screw from the front cover. Then I took an adjustable wrench and turned on the square and the same thing happened. So I saved the screws and now I can make new covers. The side one didn't go quite so good so I'll have to do a bit more work to it.