Ahhh Wisconsin. Gotta luv the rust.
Did you try PB-blaster? That generally works for me.
However, exhaust components do not see the usuall rusting due to the extra temperatures involved. I see 2 options:
1) Polish your tip while it is still on the car.
2) See if you can cut, drill, snap, whatever, those screws to get the tip off and then install it with new screws after cleaning out the treads.
I just found a dual chrome tip in my front yard. For fun, I was going to put it on my POS Blazer until the tailpipe fell off. Oh well.