Will tell you what I did, as it may help you:

I ordered replacement fans online. First I got wrong ones (12V) - the right ones are 5V (25mm*10mm). So I also got some of the 5V ones, since they're cheap. But after all, I actually replaced the 5V with 12 V. These 12V are obviously underpowered and therefore run at low speed/limited cooling capacity. But that also means they are completely silent. In my case the noise is an important factor, as if its too noisy my wife will complain...

What I also did to ensure enough cooling, because we're having close to 30 degrees Celcius at the moment: I ordered a very quiet and very effective Volador USB fan fan off of amazon.  <-- this fan has two speed options. Most of the time, lower speed is totally enough to keep the miners below 60 degrees. As a result I have 6 quite stable running miners that make as much noise as a desktop computer.