That's an amazing sidecar
If I'm understanding the mechanics - the two springs are supporting the sidecar body only - not contributing to the chassis/bike's suspension.
Its a single seat 'chair' so the springing rate won't be very high - max of 70Kilos for the body with luggage and 80 Kilos for the passenger (its French, not German
So you are looking for fairly light springs.
There must be companies that still make bespoke springs for the 'Classic Car' market, so it may be worth doing a 'Google'.
I hope you are going to produce an illustrated restoration thread
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH