I have a variety of taps for some clock repair I do, and they are difficult to come by without purchasing an entire set. They also break easily because of their size and temper.
Most of the time I am retapping into soft brass, and that makes it easy, but I've successfully made my own taps that have cut stainless steel. But snapping one off into a closed hole will quickly make you realize that when you previously thought you were screwed (no pun intended), you really weren't that bad off!
The thing you have going for you is that the heat sink material is a soft, castable alloy. You simply need to find a screw slightly (and I do mean slightly!) larger in diameter, and the same length as the one you are replacing. The thread pitch doesn't matter much, as you'll be creating new threads.
Get a fine, sharp - edged file, and cut a nick into the bottom of the screw. You are attempting to make a self - tapping screw. The nick you create needs to be very sharp and crisp to effectively cut the threads as it goes in. It also needs to be angled correctly so that as you turn the screw clockwise, the cutting edge you've made is 908 perpendicular to the threads. In other words, looking down at the screw from the top as a mechanical drawing, the tapping edge should have a sweeping edge like a minute hand on a watch.
The screw must be steel (harder than the tapped metal), and the tapping edge must be sharp, and very crisp.
Triangular files usually are not exceptionally sharp at their apexes. The cut off wheel from a Dremel can make a nice crisp edge for the cutter.
Choose a screw with depth to the head because you'll be putting a bit of torque on it, and it can be subject to being stripped by the driver.