This mod uses a custom 3d printed fan mount for the GameCube, which allows you to install a standard 50mm x 10mm PC fan.
The mod was created with the intention to use a quieter fan than the original system fan, but it also provides a means to replace failing parts in a system that is almost twenty years old.
It was requested by Newschool2k17 and inspired by his mod https://assemblergames.com/threads/gamecube-silent-fan-mod.67463/
The suggested fan is a Noiseblocker XS-1, but any 50mm x 10mm 12vdc fan will be compatible.
The noise blocker fan can be found on Amazon or likely any pc parts re-seller.
In addition to the fan it may be helpful to get a compatible connector to interface with the original port. These can be found at digikey with the following part numbers/links, if you want to cut the wire off of your old fan that will also work.
455-1165-ND - https://www.digikey.com/product-detail/en/PHR-2/455-1165-ND/608607
455-3082-ND - https://www.digikey.com/product-detail/en/jst-sales-america-inc/ASPHSPH24K152/455-3082-ND/6009458
WM4201-ND - https://www.digikey.com/products/en?keywords=22-23-2031
You only need one of the each of those items. The wire comes terminated on both ends, all you will need to do is cut it in half and solder it to the 3pin header.
You can also get just the crimp connector, but the only tool for crimping them properly runs $1400 USD (ouch) so getting the wire pre-terminated seems to be the best bet.
It is also possible to make the Noiseblocker fan even quieter for the most demanding of silent game console enthusiasts. The addition of a Noctua low speed resister can make the fan completely silent. (Hint this is a 51ohm 1w resistor Digikey; P51W-1BK-ND )
You can also buy this from me printed with a fan adapter cable bundled in