The heatsink has fallen off. can it be replaced.

If the heatsink has fallen because the plastic that holds the heatsink chipped, you can use a plastic glue to place it back. Make sure that you have cleaned the surface of the heatsink that touches the cpu off it's thermal grease before putting it back. Replace the thermal grease with a new coat to make sure proper heat transfer occurs. Check out some videos on you tube on how to apply the thermal grease properly for guidance.

Yes, there are several aftermarket ones.
http://www.svc.com/chipset-cooler.html
Just don't forget the thermal paste before installing it.