IIRC the leadtek TI4600 had the same big ass twin fan HSF setup as the later Leadtek TI4800 models....it would be pretty hard to source a replacement for that card....you might be better off buying a new video card.
But with that said you might be able to screw a 80mm fan to the large HSF unit, it should provide enougth air flow over the heatsink to keep it cool.