Odometer 74 911 not working
 

The speedometer is working, but the odometer is not.

What is the likely problem? What needs to be done to repair?

The little gear on the odometer had broken. While it is a DIY task, I would recommend you send it to a reapir shop for service. Search on Hollywood Speedometer. They do great work.
Also, do a search here on Pelican for lots of useful info if you try it yourself.

Cost
 

What does it cost to bebuild an odometer?

I think HS charges around $100.00. You need to take the guage apart.
Like I said, it CAN be done at home but if you bend or break something it will cost more in the long run.
Yes, I tried it and ended up sending mine out.:(