Hi Christian and welcome to the forum.
From your description, it's not clear what your problem is. Is your mast (stick of the fin) broken off or is it falling out? Is the plastic hole cracking? Do you have a digital camera that you could post a picture with? Have you had any problems getting replacement parts?
The older Mirage drives had stainless steel sprockets (cogs). Hobie has ordered some new ones especially for anyone who wants to put them on their drive. They have not arrived yet though.
Have you tried sailing yet with tour Adventure?