Epoxy glue it... or drill out the plastic remnants of the old rivet and fit a new allen head bolt. They're M4 (4mm) from memory.
The problem with these bolts is there's hundreds of them on a wheel so even though a stainless steel allen bolt would look quite saucy, metal won't do, as it'd upset the wheel weight and balance.
https://www.ebay.co.uk/itm/12-x-Bla... or alloy and the balance wont be affected
D