Contact a shutter repair outfit. Nothing absolutely specific to Rolleiflex with the design of the Synchro-Compur shutter in your camera. A place that handles large format cameras is a possibility. If you were in the US, there are places I could suggest, but I expect someone in your part of the world will recommend specific places.
On your photo, there is a small silver piece to the lower left of where this spring goes. It's a catch for the outer end of the spring itself. While reinstalling the spring on a similar shutter, that part simply cracked in half. A repair woman here in California says that she had never seen that part break in years of work on Synchro-Compurs, but that shutters seem to always find new ways to break. She sold me a replacement part. I bet finding a new main spring won't be difficult.