By 'heat sink not working' I am assuming you mean the fan is broken...
That card is ancient the fan probably wore it self out.
Either strap a spare 80mm fan on it or (preferably) upgrade your system.
If you have to spend any money trying to get it fixed, immediately stop what you are doing and think long and hard about how worthless that GPU currently is.
When simple mechanical parts start failing, it is like a sign from *insert favorite deity here* that you are long overdue for an upgrade.