Good idea- which is why I try my impact gun on it daily in both spin directions, but alas it's stuck solid. I have the concentrated nitric acid, but that's a bit extreme and the frame won't like it..
I reckon the weak acid would do it, IF I could get it in where the corrosion is. And that means braking the bolts to drop the engine and hoping that I can definitely get the remaining bolt bits out. And then you can see why I'm tempted to leave them in place and work around them can't you? Bloody scary...