Speedo broken. Need advice on how to correct mileage on the replacement cluster

My speedo is broken. I took it to the shop and was told that the guage itself is broken, nothing electrical or sensor related (my year is mech speedo anyway i believe). it will be cheaper to find a used one and just replace the whole cluster than have the dealer fix it. I just want to make sure it is possible to correct the odometer on the replacement peice to reflect my actual mileage instead of the mileage of the car it came out of. Is this possible without breaking it in the process?
 
BensBACobra,
My odometer stopped working as well. I did some research and found out that the mechanical gears inside the speedometer can break down after years of use. I found this vendor on eBay that has a fix. I have not installed mine yet but the purchase comes with a 30+ page PDF with directions on how to repair the gears. Here is the link:

http://cgi.ebay.com/ebaymotors/94-95-96-97-98-99-Ford-Mustang-Speedometer-Odometer-Fix_W0QQitemZ170030589991QQihZ007QQcategoryZ33678QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Hope this works for you.