Speedometer not working

So, I recently did a bunch of work to my bike including swapping the wheels out for a pair of OZ forged ally rims. All is working fine except the speedo. Revs show along with everything else on the dash.

How does the speed get measured on these bikes, is it from the nuts on the fronts discs or something else?

I think the sensor measures off the bolts which I guess have to be magnetic. Are the new ones Stainless? stainless is only very weakly magnetic so might not work.

If you want to use your GoPro on the road you can adjust the number of steel bolts you use

Obviously lifting up the front and spinning the wheel works to see if the speedo is working but it is hard work.