Hi there,
If you can find a small enough fan then yes, they are replaceable.
Yes, you can remove the heatsink from the chipset but a couple of things to bear in mind:
Make sure you clean off and replace the thermal grease and be careful when removing the mount fixings as they can be a bugger to replace if they are damaged. If they are damaged in the removal process an effective replacement are a couple of small plastic nuts and bolts but take care no to overtighten them.