Are they both Sunon Maglev PF92251V1-000U-S99 DC 12v 4.7W?
It's 127.43m3/h 75cfm.
I'm hoping the bottom one is slower! Dunno how I'd replace 75cfm to that heatsink!

Maaan, it will be a challenge to replace them with adequate fans

Top one is easy if it's 75cfm replace it with 2x 43cfm = 86cfm.
If the bottom fan is Sunon 2v 2500rpm, leave it as is. If it's faster we'll have to drop the hash speed to get it quiet.