The choke in 99.9% of all factory shotguns, is within the last two inches, give or take. Its no doubt a full choke now, with a 30" tube. Cutting it back even 3 inches is going to make it for all practical purpose, an open tube. Rabbits at 10 to 20 yards would be in danger, much further, the twenty does not have enough shot, and you will have large holes in that pattern. You would likely ruin less meat facts be known, if you kept your shooting close, as fewer pellets would be bunched up. Only a very few is all its going to take to "stop a charging bunny", so if he is still kicking, ring his neck and put him in the bag.
Re choking is possible, screw ins, or even a vintage Poly Choke, but either conversion would cost twice the guns existing value. Even a creative guy "jug choking" with a cylinder brake hone, will need an hour or two time spent, along with patterning to see if any results have happened.
Between a rock and a hard place, I know. I suggest selling it to a young guy on a budget, and looking for something more quail and dove like, with screw in chokes. Or one of the seemingly millions of guns out there with a vintage adjustable device on the end. Any of those guns, can be picked up for cheap, as they are way out of style.