Replacing the speedo gear is easy. Its 1 bolt and 1 clip. About 2 mins work.
Just follow the speedo cable down to the transmission. There should be a "U" shaped clip holding the cable into the transmission. It has 1 bolt. 8,10, or 12 millimeter. I can't remember which.
Once thats off tug the speedo out of the tranny gently since I have seen this assembly rot off two occasions. It may be stiff but even so don't haul on it, be gentle =>. Once out you should see the speedo gear on the assembly you just pulled out. At the base of the gear theres a clip which can be pulled off quite easily with a pair of needle nose pliers.
switch gear reverse process.