A few suggestions but not for the drum removal.
Post photos of the drum.
Post your question in the Kubota section here on the forum.
My best guess sight unseen is a rust/corrosion build up on what I guess is a splined shaft/drum. Try all the classic moves to loosen rust, your choice of penetrating oil(s), heat, tapping the drum to create some possible movement.
An old timers trick for stuck and broken bolts is candle wax melted into the joint.