Important to know is: when we say repot, we mean something more involved than just moving the plant from one pot to another. Repotting in bonsai involves removing a good deal of the old soil, trimming and organising the roots, and then putting in either the same pot (or a new one depeding on what we want) with fresh soil.
you could slip pot this tree... i.e. put it in a larger container adding soil. This will give you an opportunity to check if the tree is root bound, to see what kind of soil it is in, and to see if there are any potential problems. It also gives you the oppotunity to put it in a pot with drainage holes. If the roots are completely compacted, you may want to loosen them up very, very carefully so that they can go into the new soil. Sometimes the tree is wired to the pot to keep it stable,so if you want to take it out, be sure to cut the wire first.
Whatever you do, get rid of the moss, and the rocks.