Jeff,
Sorry I am a little confused, you are stating the magazine spring is rusted, but then you ask about "Sure Cycle" which is a action spring replacement.
Either way the guys at Sure Cycle can probably help you but I do not believe they make a "stock" SP10 sure cycle action spring kit. Jeff Hajjar, the developer of the sure cycle system, should be able to make you a custom one, but it may be more expense than your friend would like to put in it. I know he provides stainless magazine springs for various guns including Gold 10's, so he should be able to fix you up if that is what you need also.
Here's the link: http://surecycle.com/store/pc/home.asp
Good Luck and tell him Will Gurton sent you.