Worst case, buy a replacement cooler. Plenty of replacement air coolers for GPUs from companies like Arctic Cooling. You could convert it over to run off an all-in-one CPU cooler with an NZXT Kraken bracket.
But if you just want a quick fix:
https://pcpartpicker.com/product/PfXfrH/noctua-case-fan-nfa9x14
A 92mm fan, just take the old fan and shroud off, and strap this to it. (Or use screws, those extruded heatsinks are fairly easy to get a decent grip with some case screws, just thread them in between the fins)
A couple of cheaper 92mm offerings out there, but you do want a static pressure fan like that Noctua for pushing around a heatsink.
These would need to be plugged into the motherboard for control. Usually not easy to wire fans up to a GPU without a lot more information.