To fix the fan, remove your power supply from the computer. To do this, after you've got the case open and everything is exposed, disconnect your hard drive (needed), its respective IDE / SCSI cable, and your floppy cables (including power) to get at the power-supply's board-connector. There you will find a square, orange-looking plug. You need to remove that before you take the power-supply anywhere. You should be able to jiggle it upwards and out, but if it sticks, take a non-conductive screwdriver (flathead) and gently pry it loose from it's socket. (Remember, there's two pieces: the orange-looking base connected to the computer and the top of the orange-looking plug that you need to disconnect.) Don't pry the bottom that's connected to the computer, only the middle half. Then, after doing this, remove the supply entirely from the machine. (You'll need to get the power-switch rod out of the way.)
After opening the power supply, you'll see the fan with its red and black wires. (Dirty, isn't it?) Cut the red and black wires as close to the fan as possible to give yourself some splicing room. Then, remove the fan. Go to your local electronics distributor and get another one. They should have the right size, but it won't be a standard PC-type fan. (They are too wide.) When you purchase your new fan (around $15) simply resplice its connectors, install it, and rehook your power supply. (Make sure all of your hard-drive and floppy connectors are securely installed after supply installation.) That should do it.
If you feel uncomfortable about performing this type of "operation" on your Amiga, we would be happy to do it for you. If you attempt the above repair and botch it, we won't be held responsible -- if you have experience doing these types of things, you'll be ok. If you don't, it will either be a learning experience or a complete nightmare. You have been warned by common-sense.