You’ll have to remove the tiller handle to get the short shaft out but first, loosen the clamp screw in your picture while everything is together. It’s easier this way. When you slide the short shaft forwards, the small gear and its thrust washer will be free allowing the large gear to be lifted out.
I’ve tightened these up by making slightly thicker thrust washers for the small gear. Also, the plastic bushing for the large gear tends to seize onto the gear shaft wearing inside the bore as they seldom get greased. If so, the gear shaft can be cleaned up so it turns freely in the bushing and then the bushing gets epoxied back into the bracket. Using epoxy to shim it up would probably be easier than using a shim as the bushing is bell shaped in the middle. You’ll see when you get it apart.
Are the gears pointed or worn?