Save yourself the grief and use the std springs, just turn down the last 10-15mm (whatever is protruding past the peg mount) of the o/s pin to 8mm. Use a drill to turn the pin and hold it up against a file or grinding wheel. Or you could use a lathe as the last resort
You might need to stack the appropriate qty of washers to keep it all tight once you have it secured with the split pin.
Alternatively, use the 8mm pin with an o/s bush running through the peg and mount. Place a washer between the spring and the peg to keep the bush in place with spring and split pin installed.
Theres a few ideas to consider before you drive yourself nuts looking for a torsion spring that will fit for purpose.
Cheers Jason