I know several engine builders but it seems to me that if the springs are toast, they are toast. I usually love anything that has the word "free" attached but not if it is no longer functional. I can get the spring specs, (wire dia, # of coils and height) to figure the spring rate of new springs. That might get me in the ballpark but it would be nice to benefit from some first-hand experience. I am blown away by the amount of engineering/design analysis available on this site. The old Nike phrase comes to mind; Just do it!If it has, it will probably be in one of Biotect's posts. I kinda skimmed thru those as he gets pretty technical, figuring i would get to them when I had enough time to thoroughly digest it. But there is a lot of good stuff here.
On thing I noticed being mentioned more than once was the use of old valve springs for mount springs. Any engine machine shop will give away old valve springs, as they are scrap to them.