Just in case you missed it, further below in this post is a link to a joystick repair that I did for the Poly-61. I still had the original joystick so I settled on using it in the repair job. There are also some notes with the photos describing what I did. The guy I bought the assembly from doesn't have a knob-less joystick any more. He only has a complete unit. Actually I think it looks better.
http://www.ebay.co.uk/itm/ALPS-2D-10k-Potentiometer-with-handle-for-Joystick-or-Industrial-use-/330846675357?pt=LH_DefaultDomain_0&hash=item4d07ffa19dYou remove the potentiometers from the new stick and put the old ones on instead. It's fiddly bending the tabs back that hold the pots on but can be done if you take your time. I also had to mill a larger hole out of the joystick holder for the new assembly. Luckily I have a crappy milling machine in the shed but it could be Dremelled.