I suppose this is a long shot, but you might be able to make them out of steel strapping or other similar spring steel.
I modified an FS30 to water cooling last winter and it worked fine for a short time. Then refused to start with fuel spraying out of the carb. The problem was a broken intake valve spring. I was able to find something similar albeit with a softer spring rate and it has worked perfectly since. You have nothing to lose by attempting to make your own parts.