I had the same issue. Here is how I fixed mine (a BPT clone).
1) Loosen the set screw that holds the bearing cover on at the end of the spindle.
2) Remove the bearing cover on the front of the spindle (there are 2 holes in the end for a pin spanner, I had to make my own).
3) Rotate the spindle until you see a set screw in the side, if there is just a threaded hole, well, that is where the pin _should_be
Mine was backed out, not missing or broken. The setup is basically a dog point setscrew, and then a regular cup point set screw behind it to lock it in place. Screw the dog point in, and fit a collet. Once you are satisfied with the fit, lock down the second setscrew. Then replace the bearing cover and lock it in place.