an idea that might work .
hydrobost brakes have a return spring on the brake pedal and have a tab that slides over the brake rod mount pin and then the spring hooks to it and to the hanger frame .
you might be able to use these parts to rig up a good return spring setup if no replacement springs are available.
also in my home town there is a spring company that makes TONS of springs from super tinny to big . they might be able to help i dont know . but they would prob want to see a good one to know the shape / size wire / tension used .
http://www.fennellspring.com/