Buy a used cluster. That's probably the least expensive way of fixing your problem. Check ebay for exapmle.
You can have yours repaired if you want. There are many companies that offer this service. Here are some places to check out:
http://www.paspeedo.com/mercedes.htm
http://www.speedometerplus.com/
http://www.speedometer.com/
The other thing is that you may need new gears. In that case, you can contact a company like this one:
http://www.odometergears.com/
Good Luck.