What WICruiser said about welding spring steel. But with that type of spring you might slide (if it holds), but the spring will likely be a little weaker and your window may not stay up. I think your only real fix on the broken spring is find another original regulator or get one of the repops that are available (with spring). Kind of pricey for just the spring you need. Someone like
McMaster-Carr might have the correct wind-up spring.
