Hi Stefan, I used to make anti-roll bars for my race cars from thick wall steel hydraulic tube. This is readily available in 3m lengths and not too difficult to bend. I reckon you could make reasonable torsion springs for a Cyclekart from this. I can dig out some formulae if your interested.

https://www.leespring.co.uk

Thanks, excellent range.   £5+ per spring put me off but can’t see an alternative.

Stefan, have a look at the cheap little hand springy clamps, they usually come in sets of several different sized pairs. I salvaged a couple to give a spring return on brake and throttle pedals, they work 'perfect'