I used "Reaper Rivets" from a farm shop on a husky I was restoring. I think they are used for holding flail blades onto something, close to 5mm Dia, 20mm long and just cut them down to right length.
I struggled to find the right Dia/length/CSK combination anywhere. Rivets are mainly an aviation thing these days 100/130? csk angle.
5/32 would be a better fit for 4.2mm hole, ideally you would want a 4mm metric rivet. Remember to get a rivet long enough to allow proper forming of the head, typically stack thickness +1.5x dia.
There should enough material to open up the hole a little to allow for a bigger rivet or re-cut the csk to suit something you can get.
EDIT#Never been inside the gasgas clutch so not sure how the rivets are....