Do you have the broken parts of the original spring, or was it lost?
If it was lost, do you know someone who has a similar gun with an intact spring?
If you can answer "Yes" to the question of whether parts of the spring or another example of it is available, any gunsmith worth his salt could make a copy for you.
Heck, if the spring is made of wire, I could do it. So could you.
Click on: http://stevespages.com/ipb-sig-230.html
Look at part #30.
(No, I'm not that Steve.)