Check out this website....Jeff and Darren are very helpful and know lots about Porsche speedos. Their website gives lots of pics and has detailed instructions.
As I recall, the gear I needed was less than $30 delivered.
I also have an 85 911...I have fixed two 85 odometers now...just go slowly, and be patient. Not too bad a job, but worse than installing a Seine shifter kit.
http://odometergears.com/porsche.html