on the tube itself you will find a label with something like A66EAK552X11,that tells you exactly the tube size and type.
if swapping a tube is beyond your means i know people in the u.k that can do it for you for a price,but i must say getting a new tube and having it installed is gonna be a lot less than buying a complete new monitor.
yes you are correct about the chassis(polo2) being able to fit to many different tubes.
the thing is for me its piece of pxxx doing this job but for a novice its not so easy and there are high voltages present in a tv tube.
i think the most logical way forward for you is to get a direct replacement tube(delivered direct) and have a pro come and fit it