I've used small zip-ties in place of things like that. You actually have to use 2 zip ties. One will go through the hole from the top and then you use the top (the part you stick the tie into) on the backside and click it tight the correct amount. Then trim off the excess of the tie that was fed through the front to the back of the card.

You might have to use it on both holes as I am not sure if you will be able to get the tightness just right to hold the heatsink flat on the GPU core if you just use it on the one side.

I never really liked those push=pin hold downs as they really don't hold the heatsink in place very well and definitely not enough for optimum cooling.

Thank you for the advice, well it seems a solution even if not very clean...
What about an aftermarket heatsink?
Something like that: https://www.ebay.it/itm/VGA-Video-Card-Cooler … PB/223502547907

That aftermarket cooler should work just fine.

Grab some of these, you might need some more at some point.

https://www.aliexpress.com/item/10Pcs-lot-Com … 794388201.html?

That's another solution.
For now I used a push pin that I took from the heatsink of a gf4mx, I used it and the card works fine, tested with various 3dmark versions.
At least I know the card works while I wait for a definitive solution.
Thanks for all suggestions