If you pull air from the basement, it’s going to get replaced with warmer, wetter air from upstairs. As it cools the air is going to deposit that water on the coolest surfaces. You might be trading this problem for another, more damaging one.
In US the most common type of seal is a wax ring. The wax deforms into an exact fit. But everything needs to be extremely rigid, any movement later will break the seal.
I’ve also used a rubber seal (once) with good results.
Wouldn’t grinding the ceramic remove the enamel coating? I would worry about your future seal not fitting snug against whatever surface you end up with.
If you have to replace it, there are so many things you can do with an old toilet. People dump them in the woods, then shoot at them with shotguns, shoot at them with rifles, shoot at them with pistols. Filling the tanks with ice makes a good beer cooler for backyard barbecues. They make classy planters for flowers (usually petunias) in your front yard. So many options…
I think Aquarius would be most relevant in this situation.