What I do is just place a 50mm fan on top of the stock cooler. Because I just use test benches, I don't screw the fan on, so it's more a temporary thing. But the airflow helps a lot.

I used to have an NF7-S v2.0 and later an AN7. Those are both different model boards but I think your board still has the same mounting holes for the chipset heatsink.
Both these boards originally came with an ABit branded active heatsink/fan but it's so unreliable that I quickly stopped using them. I definitely do not recommend acquiring one. It will run for about 6mos-1yr and then quit, and the "heatsink" component is little more than a fan holder so it won't do much good by itself.

What I did was install one of those little green heatsinks that were common on chipsets of the 440BX era. Then I screwed a quality 40mm fan on top of it. This is how I ran the AN7 when it was my main PC for a couple of years, running a Barton chip at DDR400. It was (and still is) a very stable and happy machine.
I did the same with the NF7-S, it was given to a relative who gamed on it pretty much all the time at DDR333. It was also reliable.

If your existing passive heatsink will hold onto a screw, then I'd just leave it there and put a fan on top. Otherwise, if you have any scrap heatsinks around maybe one of them will fit and hold onto a fan.
If you have to buy something then I don't have a particular recommendation other than try to get a heatsink that will hold a standard fan, so you aren't stuck with the one they give you. Something with a substantial heatsink is desirable since it won't need as much fan to achieve the desired cooling, and it won't be as big a deal if the fan dies.

I also have a NF7-S, the previously owner put a passive Zalman chipset cooler on it. I added a fan and the board works great.

I used this passive cooler for my Abit http://www.acousticpc.com/zalman_zm_nb47j_nb_heatsink.html and seemd to be enough.

What about this one? I think its fairly substantial 🤣