If you're talking about the lock screw for the drum, any screw of the same diameter and thread pitch will do that job.
HOWEVER, a different screw MIGHT throw off the weight of the balance beam, and you should use a set of check weights to see if the calibration is changed by a replacement lock screw.
If it is, this is not an insurmountable problem, you just simply have to figure out the difference in the scale setting vs its indicator scale to get the weight desired.
I would recommend that once you know the amount of "offset" needed (if any) you make a note of that, and tape/glue a label to the scale base informing the world how much the scale reads "off", so you see it, every time you (or anyone else) uses the scale.
