Here is a professional hand tool, often used to remove dings or gouges in the crown. It is called Hand crowning tool, or hand muzzle canfering tool.
If you must do it yourself, and it is
more then just pits. Then this can be got, caliber specific for about a hundred.
I have rented one for a chop down I did once, but, not sure if you still can or if it would be cheaper to just pay a smith.
That is if you can find a good one in your area. In my area we have a lot of armorers, but all the good smiths have either died, went gun brand or type specific; or retired.
But if it is real bad. Like where you need to cut it. I would have to go with Bret on the gunsmith with a lathe thing. Even if you have to mail it, or part of it.
I have shortened a barrel and crowned it with this type of cutter before, worked out real well. But I was taking a chance with a $75 dollar Sears and Roebuck gun.