That's a shame!

Obviously, the best option would be to recone it. But if that's not an option for you, I've had good luck using white glue. I usually limit these kinds of repairs to speakers that will be used for less critical listening. But it actually works pretty well.

Just put a bead of white glue on your finger and dip your finger in a glass of water. Rub the bead and water together between a couple of fingers top mix it into a thinner consistency. Then apply it along the tear, working it into the fibers. Don't make the paper cone too wet because it can shrink and warp when it dries. But make the consistency of the white glue thin enough it is easily absorbed into the paper surrounding the tear.

Bring the edges together as perfectly as possible. If the repair requires a lot of glue, you might put some on the opposing diameter to balance the weight. A thin film doesn't weigh much when dry, but to achieve near balance would still probably be a good idea. The motor provides force in the center, so an imbalanced weight load would cause a slight torque moment.