That is the Volvo MS2 transmission. The seal is easy to replace but you may find the surface that it runs on has some rust which is what damaged the seal in the first place.
Remove the screw at the center of the gear and then the gear simply slides off. It will turn a little when you back it off as the gears are helical cut. The gear used to be horrendously expensive about 15 years ago. We solved the problem by machining the gear down a few thou and then fitting a sleeve over the gear to provide a good surface for the seal to ride on. This is called a speedi-sleeve and you can read about it here
Speedi-Sleeve shaft repair kits