As I said, I don't see options to wind a heavier gauge than what they used, on such a narrow core. I am not able to find any heavier springs commercially and I am not able to wind my own.
It appears to be a 0,8 mm wire wound to a 5 mm outside diameter, meaning the core has to be about 3 mm or maybe even narrower than that to end up with that outer diameter.
I am stuck with that outer diameter, since the spring is in a bore inside the leg. Using 1 mm wire would only require an even narrower core.
I have no means to do that without overstressing the wire.