Those side plates are a really tight fit over the pins and would probably stay put even if they weren't mushroomed out at the end (although I wouldn't advocate trying it!). The point is that you wouldn't need to flare out the pins very much to get a secure join.
Couldn't you use something as a suitable anvil on the back of the sprocket and then peen over the ends of the pins with a small hammer and/or drift? You could use those spacers to prevent the side plates closing up too much.
Alternatively, assuming the pin diameters and side plate thicknesses are the same, you could fit a hollow rivet link from another manufacturer. For example, I believe DID pins are 5-5.1mm diameter.
https://www.ebay.co.uk/itm/DID-Chain...J/283292492327