Well. You do have a bit of a situation there. For future builds take that tube of CA, find a nice high shelf, put it up there and forget it exists. Seriously, all you need to build balsa and paper LPR rockets is plain old white glue - Elmer’s Glue All works fine though if you want to start a lively discussion amongst rocketeers just ask the which adhesive is best question
Now back to your problem. The Mini Mean Machine is a very stable and forgiving design, you probably
could just wrap some masking tape around the exposed motor mount tube and, maybe, have a safe flight. Doubtful that the range safety office (RSO) at a club event would allow you to fly it but there’s a better than 50/50 chance it’ll fly ok in your backyard. Frankly though, that’s not good enough so let’s see if we can fix it.
If I had this happen I would, carefully, cut the fins off, sand the attachment points smooth, take a length of BT-20 tubing from my scrap box, cut a piece to cover the exposed mount, cut another section - make sure this section is as long as the root edge of your fins, slot that section so it can slide over the new aft tube and the existing body tube like a sleeve, glue the new bits up with ordinary white glue, carefully sand the root edges of your sliced off fins until all of them are even and square, then, again with plain white glue, reattach the fins ensuring that you space them evenly and avoid the slot of your tube “sleeve”.
Since you probably
don’t have (multiple) boxes of random rocket parts here’s an option - take the length of tube you need from the
forward end of your rocket. Yes, it’ll make it a bit shorter, but as I said earlier, the MMM is a very stable design - shortening it a few inches won’t affect how it flies.
Model rocketry is all about problem solving - and when you’re done you have something
you made that does something - fly into the air with fire and smoke! How cool is that?!