Look at the 3 digit number on the top.  That tells you the ohms rating.  For example, the unbroken one in the photo has 204.  Thats 2-0-0000 ohms, so 200000, or 200K ohms.  Then just find a pot that is the same size to fit where the broken one is and solder it in.  Unfortunately, you'll need to know what it was adjusted to.  I can't help you with that, you'll need to find a schematic or something.  IF the broken one hasn't been accidentally changed from handling it, then you may be able to measure it with a DMM and adjust the new one to the same setting.