just thought I post the solution we came up with
we made a new step out of stainless steel
SS being roughly twice the strength of mild steel means the replacement can be much thinner, so the base plate of the new step is just 6mm thick. This allows that the studs are now able to be threaded for a fastener that will have enough grip.
Here the studs are run off with a 7/16" NC button die that will take a conventional nut and washer on the forward pair, but may have to make some special custom nut for the aft pair as the grip is still a tad too shallow. Envisage doing this out of scrap SS plate.
My thanks for people helping to find a solution.