Hmm...
Then I'm afraid I'm out of ideas...
Since I've never removed the Heatsink off any Card, I can't really help with it.
But once the Screws/Connectors that hold it in place are removed, it might be already possible to take it off.
If it is still rugged and fixed by a adhesive glue/thermal paste compount, I've heard people taking it into a sealed bag, and leave it in a freezer for some time.
Once the Card is extremely cold, the Adhesive Paste should become unflexible, and (supposedly) it can be then removed with a careful, yet tight twisting movement... The sealed bag is needed to prevent moisture to form/freeze on its critical, volunerable components.
But anyway, I wouldn't recommend it. There were experiments to replace it with an Orb cooler, and some people found out it cooled actually worse than the stock Heatsink
The best thing that was usually done, was to take a really good 40x40mm 486-class Fan and screw it onto the Heatsink.
Since the Card gets quite hot, most 0815 486-Fans don't live long on it, hence it really needs to be a good one.
With Plastic strips, it should also be possible to attach a more potent 60x60mm 586-class Fan onto it.
Besides much more elaborate constructions, that should be the cheapest, and about most reliable/safe way to increase cooling by a significant margin...