I did a few temporary fixes on one once upon a time, but the problem is that gas will go through as well. If you price toilets, they aren't that much. Maybe it's the seal, but I think more likely it's got a crack.
If I was you, I'd buy a new toilet. The work to replace/repair is close to the same. You also can potentially get a more appropriate height since toilets come in different heights.
Oh, and flush the black tank with fresh water before removing the toilet. Doing that will keep the odor down.
Good luck.
